TF. M. SutcMngs — Clay^, Slates, mid Contact-Metamorphism. 41 



regrets every hour wasted in using the old rotating-stage horror ! But 

 the full advantage of the change for the special class of vfork iu 

 question was only realized when my friend Mr. Dick persuaded me 

 to use a -iV inch oil-immersion objective, a thing that would have 

 been quite out of the question with the rotating-stage. The optic 

 figures obtained by its use (of course in combination with a suitable 

 condenser, and with a sufficiently small perforation in the diaphragm 

 under the eye-piece) are quite good and certain where those got by 

 means of a high-angled ^ inch are too indistinct to be of any value, 

 or where the mineral under observation is so small in area, and so 

 thin in section, that the \ inch gives no figure at all. 



Thus, these contact-slates have to be sliced extremely thin in order 

 that there may not be any superposition of minerals, and iu these very 

 thin sections nothing but optic figures in convergent light can usually 

 safely decide whether given grains of the mosaic are quartz or 

 felspar, the felspar-grains being without cleavage.^ 



There are also very great advantages in using these lenses for 

 other special purposes outside of the study of optic figures, as for 

 the examination of minute structures, enclosures, etc. Those 

 petrologists who are using these high-power immersion-lenses (there 

 is no need to stop at -iV inch) will, I think, fully concur in what I 

 have said above, and in the desirability of encouraging their use by 

 others who may still regard them as rather outside the requirements 

 of the petrological mici'oscope. 



In the altered slates at Shap, then, felspar is plentiful. In some 

 slides there appears to be almost as much felspar as quartz in 

 the limpid mosaic. The grains of it, like those of the quartz, are 

 rich in enclosed microlites and have frequently the more or less 

 sharply bounded polygonal outlines so characteristic of both these 

 minerals in fully developed mosaics of contact-rocks. This mosaic, 

 or " tesselated " structure, when at its best, is so very like the cells 

 of a honeycomb that in German literature it is frequently spoken of 

 as " honeycomb-like." This is such a very good description of it 

 that it is almost a pity the word is already appropriated among us 

 for another meaning. 



I have made the following two analyses to show the composition 

 of the "flags" of Wasdale Beck. The same remarks apply here, 

 as regards the non-determination of titanic acid, and the statement 

 of all the iron as ferric oxide, as in the case of the analyses of clays 

 already given : — (Analyses on p. 42.) 



J. is a sample of the rock exposed in Wasdale Beck, near the 



^ 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 vohmie. 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, in which 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 case where grains of this sort are seen in a mosaic, where a good deal 

 of felspar is not also present in an unstriated condition. 



