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Published online by Cambridge University Press: 01 May 2009
MR. Teall,inhis very interesting and suggestive address to the Geological Section of the British Association at Nottingham, alluded to the composition of sediments (clays, etc.) as compared with that of older sedimentary rocks, and specially to the amount of alkali contained in them.
At the time he made his remarks I was engaged on some analyses of clays, with a view to gaining information on this particular point,inconnection with studies of clays and slates which I have been carrying on for some years, and on which I have published papers in this MAGAZINE on former occasions. The results of these analyses, and some considerations arising out of them, I propose to give as the first item of these present “notes.”
page 41 note 1 There is an appearance to be seen in these newly-formed felspars which is of the greatest diagnostic value. This is a peculiar striation, best seen in polarized light. It does not seem to be either a cleavage or a twinning. It appears to be peculiarly characteristic of felspar formed at contacts, and has been so noted by observers. It is duly emphasized by Zirkel in his latest volume. Unfortunately it is not regular in its appearance, and there are plenty of mosaics, especially those of smaller grain, with a good proportion of felspar,inwhich it is not seen. Where it does appear it may be relied upon as evidence at once. All grains showing it are felspar, and I have not come across any ease where grains of this sort are seen in a mosaic, where a good deal of felspar is not also present in an unstriated condition.
page 43 note 1 In some contact-rocks the felspar is so developed that it is only a question of a glance through the microscope, even with low powers, to make sure of its presence in quantity and that it is a new formation. Thus,insome of the contact-slates and grits (greywackes) of the Kibe Valley and the Lausitz district of Saxony there are large grains of well-cleaved orthoclase and well-twinned plagioclase, whose enclosures of biotite, quartz, etc., place their nature as contact-minerals beyond question. It is in the limpid and often fine-grained “ mosaic” of such rocks that the presence of felspar appears to have been often overlooked, and in which, nevertheless, I consider its presence can nearly always he demonstrated.
It may be of use to mention that a good series of specimens of the most interesting contact-rocks alluded to above, from Saxony, can be obtained from the mineral depot it the Mining College at Freiberg.