A Small series of rock-specimens, consisting chiefly of fragments of basalt, dolerite, volcanic conglomerate and tuff, which were collected near Kilima-njaro, by F. Holmwood, Esq., C.B., lateConsul-General at Zanzibar, was recently forwarded to the Museum at Jermyn Street, and placed by Dr. A. Geikie in my hands for determination. Among these specimens was a small block, the crystalline texture and high density of which at once arrested attention Freshly fractured surfaces of this rock present numerous glistening facets, which with the aid of the pocket-lens are seen to be the cleavagefaces of a yellowish-green, a dark-green, and a garnet-red mineral. These minerals are respectively olivine, hornblende, and hypersthene in nearly equal proportions, the hornblende being perhaps slightly predominant. They form a holocrystalline granular aggregate, in which the.grains are of nearly equal size and without crystalline contours; in other words, the structure is allotriomorphic-granular. Subordinate both in size and quantity to the three principal constituents is magnetic iron-ore.

The Memoir of the Geological Survey on “The Geology of Part of East Lincolnshire,” by A. J. Jukes-Browne, lately reviewed in the GeologicalMagazine, proves so interesting that I am tempted to compare the classification of the Glacial deposits which he has adopted with that I have proposed for the Midland Counties. There are also some features in the distribution and lithological characters of the Boulder Clays to which I should like to draw attention.

Mynydd Mawr, about three miles west of Snowdon, is an abrupt rounded hill, 2300 feet high, separating the valleys of Nantlle and Cwellyn. A reference to the maps of the Geological Survey (75 N.E. and N.W.) shows it to be due to an isolated boss of “intrusive hornblende-porphyry ” in the form of a rounded parallelogram, a mile and a half in its longest diagonal. Dr. Hicks has mapped this patch as Pre-Cambrian, and included it in his Arvonian system; but apart from its position, breaking through a regular succession of Cambrian (or Ordovician) rocks, an examination of the junction affords convincing proof of its intrusive character. On the northern flanks of the hill the relations are well exhibited, and the induration and extensive mineralogical alteration of the slate in the vicinity of the boss are very marked. It is indeed by no means easy to determine on the ground the precise point where the hardened, semicrystalline slate gives place to the porphyry.

In writing Part I. of the British Museum Catalogue of Fossil Reptilia, in which a large number of forms are known solely by disjecta membra, I was fully prepared to find that many of my determinations, which are frequently provisional, would subsequently need correction. It is, moreover, highly probable that there are really more species represented in the collection than I have indicated, as in some cases, in my desire to avoid the introduction of new specific or generic names which could not be fully substantiated, I have provisionally included under generic or specific headings specimens which may prove to indicate distinct forms.

I Presume that most English students of Mesozoic Reptiles are acquainted, at least by report, with the magnificent collection of the remains of Sauropterygians and other Saurians from the Oxford Clay of Northamptonshire in the possession of Mr. A. N. Leeds, of Eyebury, near Peterborough. Those, however, who have not had the good fortune to see this unrivalled collection, can have no idea of its richness, or of the light it throws on the organization and affinities of the Sauropterygians of the later Jurassic seas.

Subsidences are by no means unusual amongst the arable lands in the Chalk districts of this county. No one, who has travelled in these parts, can have failed to notice the numerous holes and dells dotted about the fields in all directions, many of which have fallen in, or will do so,incourse of time. Nearly all of them mark spots where the Chalk has been dug up for chalking the land, which is then said to work better.

There are two facts which are closely connected with the present I condition of the Alps—(1) that the rainfall on the Alpine slopes on the Italian side of the watershed, as a rule, is heavier than that on the other side, and (2) that the valleys also are steeper. It is indeed true that the distance from the watershed of the Central Pennines to the lakes on either side, north or south, is not very different, but the level of those on the latter is some 700 feet below that of those on the northern. This difference of level is especially marked in the upper part of the valleys, places correspondingly situated being always lower on the southern than on the northern face; for instance, Zermatt is rather above Macugnaga, but as the crow flies it is more than double the distance from the watershed.

About nine years ago I contributed to the pages of this Magazine, an account of the two above-named volcanoes, which was founded on observations made during brief visits by myself in the year 1873, and on the published records of visits by previous observers. My present object is to draw attention to information which has since then been acquired regarding them.

Many of the rocks of this district have been already described by the late Mr. E. B. Tawney, in a series of papers entitled “Woodwardian Laboratory Notes,” contributed to the Geological Magazine a few years ago. The following additional remarks are to a certain extent supplemental to the above-mentioned papers, and are founded on a microscopical examination of a large series of rocks, collected some few years ago during a brief visit to the Lleyn district.

More than forty years ago the detached teeth of Hybodont Sharks were recognized by Reuss in the Cretaceous rocks of Bohemia, and these were referred to the genus Hybodus under no less than eight specific names. About twenty years later, evidence of a somewhat similar Selachian was discovered by Mr. William Davies and Mr. S. J. Mackie, in the Lower Chalk of Kent; and the cartilages of the jaw, with a few teeth, were briefly described by the last-named geologist under the name of Hybodus dubrisiensis. In 1886, the present writer pointed out, from more recently discovered specimens, that the English Cretaceous species was more specialized in every respect than any of the typical forms of Hybodus; and quite lately it has been proposed to regard this fossil as generically distinct, with the new name of Synechodus.

Elsewhere I have shown that metals under certain conditions when subjected to changes of temperature undergo permanent deformation. Thus sheets of lead ridge up and fold even under the influence of atmospheric changes, as may be seen by the examination of any old lead flats or gutters. The cumulative effect of small but repeated changes of temperature is very striking; and I have used it in illustration of what I conceive to be the true explanation of the ridgings up of the earth's crust called mountain ranges. The examples given of the effects of alternations of temperature are mostly in metals, but I have also shown that other materials not ductile are affected in the same way. in a lesser degree.

I.—To avoid useless repetitions, I shall consider as admitted what I have said in my paper “Sur le genre Euclastes,” especially:

1. Euclastes, Cope = Chelone, Owen (non Ritg.) (pars) = Lytoloma, Cope = Puppigerus, Cope (pars) = Glossochelys, Seeley = Pachyrhynchus, Dollo = Erquelinnesia, Dollo.

The following important and interesting notes on avalanches and the dangerous wind-blasts they cause, appeared in the Pall Mall Gazette for February 28th, 1888. As we do not remember to have seen the geological effects of these phenomena described in any detail in our numerous geological text-books, we believe Mr. Symonds' notes on the subject will be of much interest to readers of the GeologicalMagazine.

In “The Palæozoic Fossils of Canada,” vol. i. p. 3, the late Mr. Billings stated that by treating a silicified specimen of Archæocyathus Minganensis with acid, he ascertained that it contained numerous siliceous spicula—slender, fusiform, slightly curved, acute at both extremities—and that these fossils must therefore be classified among the extinct tribes of sponges. Referring again to this species in the latter part of the same volume (pp. 354–357), this author states that he had found the same spicula present in great numbers in another large species, Trichospongia sericea, and that it therefore remained an open question whether they actually form part of the structure of Archæocyathus, or belonged to Trichospongia.

The effect of the attraction of continental land upon the oceanic waters adjoining seems to have been very much overlooked by British physical geographers. That some slight effect arises in the direction of elevating the surface of the ocean in proximity to the coast is generally admitted, but the amount of rise is considered to be small, perhaps insignificant. The prevalence of these views was attributed by the author to the widespread influence of Lyell's hypothesis of the uniformity of the ocean-surface all over the globe.

The crystals of felspar are found to be most numerous in those rocks which lie in the closest proximity to the gabbro and serpentine; they have their long axis at various angles, and are mostly small, except at Parn Voose, Cavouga, and Green Saddle. The felspathic and hornblendic lines often circle round the crystals.

Such is a brief statement of the bare facts of our observations in this particular direction. Without discussing any theory as to the true nature and origin of the whole of the schists, we think that the porphyritic structure, so prevalent in the dark bands of the “Granulitic Group,” in many of the micaceous and other rocks, as also in the later intrusions cutting the serpentine, indicates an igneous origin for many rocks hitherto regarded as schists.

In some cases it is of practical importance to petrologists to estimate the strength of the double-refraction exhibited by doubly-refracting minerals in thin slices of rock under the microscope.

Babinet's compensator, a description of which will be found in Rosenbusch's Microskopische Physiographic der Mineralien und Gesteine, affords a means of accurately calculating the refractive indices of the rays into which light is divided in its passage through doublyrefracting crystals. Michael-Lévy also devised another exact method based on observations made on the tints presented by slices of doublyrefracting minerals, an account of which appeared in the Bulletin de la Societe Mineralogique de France, and which is discussed in a work recently published by Michael-Lévy and Lacroix.

Now that mapping constitutes such an essential part of field-work, it may be of use to some of your readers to connect together a few rules which have occurred to me on this subject.

Valley-Outcrops.—Professor Green has devised an admirably common sense method by which the outcrop of a flat rock-bed can be traced by means of contour lines. The main results gained from this method may be thus summed up:—

In the Museum of Practical Geology, London, is a remarkable specimen, marked , which the late Mr. J. W. Salter referred to, in the ‘Quart. Journ. Geol. Soc’ vol. xix. 1863, p. 92, as “a huge bivalve Crustacean,” … “with a carapace 7 inches long,” giving it the name “Dithyrocaris pholadomya.” It is in a dark micaceous sandstone, from the “Carboniferous Shales” (or Lower Limestone of the Carboniferous-Limestone series), of Berwick-upnn-Tweed. In the “Catalogue of the Collection of Fossils in the M.P 6.,” 1865, p. 116, it is referred to as “Dithyroearis pholadiformis”; but whether or no this specific name was an alteration made by Mr. Salter himself is doubtful.

Considering the interest that has been awakened of late in the Bagshot Beds of the London Basin, and the paucity of good sections open to the light of day exhibiting any considerable vertical range of those beds, it has occurred to me that a fuller description of these Finchampstead sections may be of sufficient interest to students of Tertiary geology to justify its appearance in the pages of the Geological Magazine. In the task we have before us of attempting to workout the old physical geography of the Lower Thames Basin in later Eocene times, every contribution of facts (by no means the easiest part of inductive science) must be welcome to students of the subject. The problem was sketched in its outlines and bequeathed to his successors by the versatile mind of the late Prof. John Phillips, F.R.S. “In considering these remarkable strata (he says, of the London Bagshot Beds), which were accumulated in a period so near, geologically speaking, to our own, we are presented with problems of great interest, which, if they can be solved, will have more than local application.