Many years ago, and before I entered on my duties in the Edinburgh Museum, the late Mr. C. W. Peach pointed out to me a specimen in the Hugh Miller Collection, which he was inclined to consider as new. It was a Cromarty nodule with dislocated remains, including a dorsal and a pectoral spine, of what was apparently a Diplacanthns of unusual size.

Calcareous algae in limestones of the Permian Chihsia Formation near Nanjing, China, are preferentially replaced by quartz. Replacement postdates both cementation by sparry calcite and also neomorphic alteration of the original skeletons to sparry calcite. It is thus diagenetically relatively late. The original replacement silica could have been massive opal CT, opal CT lepispheres, or quartz. Lepisphere-like bodies occur in a few specimens. Megaquartz rims represent later overgrowths on the silicified skeletons. Although the original ultrastructure of the algal skeletons is not preserved, the preferential silicification allows fine details of the skeletal morphology of the algae to be observed and the distribution of silica indicates the style of replacement which has probably occurred. In some specimens it is possible that dissolution of the skeleton was followed by void-filling by silica, including the possibility of lepisphere formation. But in other cases in situ replacement, probably by massive opal CT or quartz, is reflected by the replacement of the outer parts of calcite crystals by silica. This process produces a network of silica, and as it proceeds the calcite centres of the grains become progressively smaller until silica replacement is complete. Sponge spicules may have provided the most important silica source for replacement. It is proposed that water-insoluble organic films surrounding matrix grains, and now preserved as bituminous material, protected the matrix from replacement thus promoting selective late diagenetic silicification of these fossils.

Corallum conical and, as a rule, gently curved, though often straight; a short broad outline is not uncommon. The epitheca has well-marked longitudinal ribbing, sometimes obscured by fine annular striations in the neighbourhood of the calyx; slight constrictions of growth may occur, but there is never an interrution in the continuity of the epitheca. Good figures are given both by Milne-Edwards & Haime and by de Koninck.

The Christianburg and Akyma deposits afford excellent examples of the highly aluminous or bauxitic type of laterite formation. They here occur as parts of a residuary deposit derived from the decomposition in situ of an igneous rock, probably a porphyrite or a tuff, an almost pure quartz-sand, and masses of bauxite containing in the more aluminous varieties from 92·6 to 94·4 per cent. of the hydrates of alumina and in the ferruginous ones 64·6 per cent. of the hydrates of alumina with about 24 per cent. of the oxides and hydrates of iron.

It has long been recognized that, in early Eocene times, while the “Calcaire de Mons” was being deposited in the Belgian area, large tracts of the Upper Chalk were removed by denudation from the south-east of England. The true extent of the unconformity between the Chalk and our oldest Eocene strata is now to be seen only where the Eocene cover yet remains. Even where the Chalk is so covered, the unconformity must have been accentuated to some extent in consequence of the solvent action of percolating carbonated waters, the evidence of which is familiar to us as “pipes” in the Chalk filled with Eocene strata, and the persistent bed of unworn, green-coated flints (“Bull-head” Bed) which everywhere occurs between our lowest Eocenes and the Chalk; but such secondary action is likely to have been more or less uniform throughout the whole area beneath the Tertiary cover.

The appearance of Professor E. Weinschenk's paper at the close of last year on two specimens of Moldavite from Kuttenburg, in Bohemia, affords a convenient opportunity of recording occurrences of the related ‘obsidianites’ (referred to by Professor Weinschenk as Billitonite and Australite) in the Malay Peninsula.

Many ways of measuring geological time have been attempted by various geologists of eminence; but however diverse their methods, so far as I know, all finally hinge upon either the rates of denudation or accumulation. It is urged as an objection to this that in past ages such actions have gone on more rapidly, and therefore the calculations based upon present rates are valueless. Physicists on the other hand have sought to put a limit to the age of the earth much below what geologists generally demand. Reasoning from certain data which are necessarily more or less hypothetical, they say, that from the thermal condition of the globe at present, it cannot be more than from ten to twenty million years since it was at a temperature in which life on it would have been impossible. Geologists can hardly be blamed if they attach greater weight to their own observations and data and to reasoning that is more familiar and appears more certain and satisfactory to their minds.

The very uniform trend of all these ridges will have been noted, but my previous remarks on the general strike of the foreshore chalk require considerable modification. The whole of the chalk so far exposed may be divided into four sections. Each of these comprises an exposure in or close to the cliff of what appears to be the highest part of a ridge running down the beach in a direction from 10° to 30° south of east (and sinking as it goes) to about the half-tide level. Here three of them (the exception being the brickfield chalk) turn and run for some way roughly parallel to the shoreline, and then resume their original direction and run out to sea. The brickfield chalk only varies from this plan by running out to sea with practically no change of direction on the way. Except where a ridge is running up to the cliff, the substratum of the beach is invariably glacial clay down to about half-tide level. Here it is either banked against what appear to be vertical faces of chalk or else (between the foreshore exposures.) disappears under the sand. It has never been seen to run out to sea, and every time a fresh bit of the foreshore below the half-tide level is cleared of sand it is chalk that is revealed. In the case of the section attached to the north bluff there is below the half-tide level a continuous mass of chalk with perfectly regular bedding exposed for at least 1,000 yards along the shore, and directly opposite the north bluff I have myself seen chalk continuous from the foot of the bluff for over 200 yards straight out to sea (except about 20 yards close up to the bluff, which, however, are covered by Mr. B. B. Woodward's letter in the October (1905) number of the GeologicalMagazine), the regular sequence of the beds being only broken by one fault. The section attached to the ridge coated with a sheet of flint is a good second in size, showing chalk in regular sequence for a length of over 400 yards, and maximum (exposed) breadth of about 45 yards.

In a paper entitled “Observations on Belemnites,” which was communicated to the Geological Society of London in April, 1823, J. S. Miller defined the genus Belemnites thus: —“A cephalopodous? molluscous animal provided with a fibrous spathose conical shell, divided by transverse concave septa into separate cells or chambers connected by a siphuncle; and inserted into a laminar, solid, fibrous, spathose, subconical or fusiform body extending beyond it, and forming a protecting sheath.” In May of the same year Mr. Miller contributed to the same Society another paper in which he instituted the genus Actinocamax for “spathose bodies which resemble the belemnitic guard in general appearance, but are distinguished from it by presenting, instead of the terminal cavity intended for the reception of the chambered shell, a protruding and convex base.”

Dolomite is common in parts of the Chalk Group of North Sea petroleum reservoirs. The stratigraphic distribution, microtextures, and stable isotope ratios of dolomite vary both within and between four of the North Sea chalk fields examined in this study, the Eldfisk and Tor of the Norwegian Sector, the Dan Field of the Danish Sector, and the Machar Field of the United Kingdom Sector, indicating that the conditions favourable for dolomitization occurred at different times and places in the North Sea Central Graben. Dolomitization in all of the examined fields occurred late during diagenesis, after significant compactional grain breakage. Dolomite precipitation in the Eldfisk, Dan, and Machar fields occurred in modified sea water, which had Σ18O values between the assumed Cretaceous/Tertiary seawater value −1 and +8‰ (SMOW). The enrichment in 18O was probably the product of calcite recrystallization at elevated temperatures in a low water/rock ratio system. The timing of dolomitization varied with respect to organic diagenesis; dolomite precipitation in the Eldfisk Field coincided with bacterial methanogenesis whereas dolomite precipitation in the Machar Field probably coincided with bacterial sulphate reduction. The magnesium in the dolomite may have been derived from the neomorphism of high magnesium calcite to low magnesium calcite.

The geological succession of the rocks of the N’Changa District is so typical of that of the Northern Rhodesia Copper Belt as a whole, and indeed of much of the Colony of Northern Rhodesia, that it is felt that a summary of the sequence of events which together make up the geological history of the district may serve as a contribution to the geological literature of the general Northern Rhodesian field. For a fuller account of the details of the petrography and mineragraphy of the rocks and ores of the N’Changa District reference should be made to Bancroft and Pelletier (1929), and Jackson (1932).

Some three years ago, when travelling in Devonshire, I had the opportunity of visiting some places on Dartmoor, and of collecting on the border of the Dartmoor Granite some specimens of rocks, which on closer examination seemed to be of general interest. Although I have not finished the examination of these, yet I have made some remarkable observations, which I should like to describe to the readers of this Magazine.

So far as the writer is aware, no sodic or “contaminated” rocks have been recorded from Iceland, and it was thought that an examination of some black sand collected by Dr. L. Hawkes, and kindly supplied by him, might reveal the presence of minerals from such rocks.

So little is as yet known of the Palæozoic Ophiuroidea, that the discovery of some specimens that represent a new species of Protaster in the Upper Silurian rocks of Victoria is of interest. Some of the specimens were forwarded to Dr. Woodward by Mr. Frederick McKnight, of 60, Hawke Street, West, Melbourne, Victoria, some time ago, but they were too fragmentary to be of much service; several better examples having been received from the same source, Dr. Woodward has kindly entrusted them to me for description.

The northern mountain range of schistose rocks of the Island of Trinidad reaches an altitude of 3,012 feet in Mount Tucutche and consists of a wild and tumbled mass of ridges with deep intervening valleys clothed from top to bottom with tropical vegetation, the abode of a practically undisturbed fauna and flora. Westwards the range is continued in the rugged and precipitous islands of Monos, Huevos, and Chacacare between which are the Bocas through which ships pass in entering the Gulf of Paria from the north. The entrance to the Gulf is famous for its scenic beauty. North-eastwards the schist-range reappears in the island of Tobago, but no limestones are reported to occur among the rocks there. Westwards the high range of schistose mountains continues along the northern coast of a great part of Venezuela,4 and over 800 miles further west at Santa Marta in the Kepublic of Colombia I have seen a series of schists very similar in lithology to those that form the northern range of Trinidad.

Four years ago M. R. D. M. Verbeek, Director of the Geological Survey of the West Coast, Sumatra, placed in the hands of my friend Prof. T. Rupert Jones, F.R.S., for description, a collection of fossil remains obtained by him in the prosecution of his geological researches in Central Sumatra, accompanied by some notes and sections of the geology of this little-known region of the globe.

ONE mile due east of Tantallon Castle, on the East Lothian coast, there stands a conspicuous tower or beacon. Its purpose is to afford an offshore warning to mariners. It has been constructed upon the seaward extremity of a northerly-extending ridge of volcanic ash or agglomerate. In recent years the rocks exposed here have been carefully examined by a well-known Edinburgh geologist, the late Mr. T. Cuthbert Day, and his results are reported in an interesting paper entitled “Volcanic Vents on the Coast, from Tantallon Castle eastwards to Peffer Sands, and at Whitberry Point” (Trans. Edin. Geol. Soc., xii, 1930, p. 213). Mr. Day thought that the ash and agglomerate lying south of the beacon represented the materials belonging to two contiguous but distinct volcanic vents of Lower Carboniferous age. To the twin orifices he applied the designation—“ The Car Double Vent.” In the northerly vent there occur two basaltic intrusions intersecting the pyroclastic rock. One of these, an irregular 4 ft. dyke, trends north-west hard by the easterly side of the beacon. The other, an oval plug covering 500 square feet, or thereby, is situated about 100 yards to the south-east. Both masses were determined petrographically by Mr. Day as “ dark red basalt with conspicuous dark crystals of augite, somewhat decomposed, but appearing under the microscope to approach a limburgitic type ”. It is in reference to these “ limburgitic ” intrusions that I present the following brief note, because, lately, I have discovered that they consist of leucite-basanite, a rock of basic felspathoidal character not previously known to occur in this country.

In 1909 I wrote a letter to the GEOLOGICAL MAGAZINE (p. 431) objecting to a statement in a review of the late Sir John Harrison's Goldfields of British Guiana that only products of weathering containing free aluminium hydroxide in hot moist climates should be considered as laterite. This letter led to contributions from the late J. W. Evans, T. Crook, and Sir John Harrison. It is particularly interesting now to read again J. W. Evans's remarks. In the volume for 1910, pp. 189, 190, he said hehad intimate acquaintance with the material to which the name laterite was first given in the area in which it was typically developed. On p. 190 he said that laterite varies, but one feature remains constant, the small amount of combined silica in proportion to the alumina present and that “it is in this respect that laterites differ from clays, which also occur as tropical decomposition products and are sometimes incorrectly described as laterites ”. On p. 381 of the same volume J. W. Evans wrote: “I admit that he (Buchanan) did not know its true chemical composition, but in spite of that it must be accepted as the type of what we ought to call laterite.”

I was led to make this analysis with the view of determining if possible whether the extreme hardness of the Irish Chalk were due to either a chemical, a mechanical, or a calorific alteration, from the influence of the overlying basalt. If it were owing to chemical change, we should expect to find a large per-centage of silicates and a diminution in the amount of lime; if to the influence of heat, carbonic acid would be driven off, silica would be in excess, and the presence of the insoluble bases, such as the peroxide of iron, oxide of manganese, and alumina, would become more apparent; while if a mechanical cause or pressure were the reason, no change would take place in the relative amount of the constituents. I believe the result seems to show that the induration of the Chalk must be set down chiefly to the last agency,—if to anything apart from the original formation of the rock,—and that the power of alteration exerted over it by the heat of the molten basalt has been small indeed. At the same time a certain addition has been made to it by means of water holding chemical bodies in solution.

From time to time I have accumulated a large number of results obtained in identifying precious stones by means of their specific gravity. From these results I have selected about 70, which will be found arranged below. The observations have been made with care, and, where no temperature is given, at 15° 5 C.; an asterisk denotes those determinations in which a very accurate assay balance by Oertling was used, and in which the specimens were immersed in alcohol, not in water. In these latter determinations any error would be confined to the third place of decimals.