166 NOTES AND MEMORANDA. [jtu'n'a\%Sn?'iwo! 



The Microscopic Structure of Rocks. — Mr. J, A. Phillips has 



sent us the reprint of a paper in the ' Philosophical Magazine ' for 

 December. In this the ai;thor, in describing certain slates, felsites, 

 and elvanites in the county of Waterford, Ireland, gives an account 

 of the structure as seen under the microscope, and the paper may 

 therefore be of interest to oiu' micro-geologists. He describes four 

 specimens. The fii'st is an elvanite of specific gravity 2 • 66 ; a section 

 of this vfas observed. Examined under a ^-inch objective, this rock, 

 says the fi,uthor, is seen to be composed of an amorphous greyish 

 matrix in which are porphyritically imbedded crystals of quartz and 

 felspar, the latter being chiefly oligoclase. In addition to these, a few 

 small crystals of some hornblendic mineral are sparingly disseminated 

 throughout the mass. It was fm-ther observed that the larger quartz- 

 crystals are sometimes penetrated by crystals both of felsj)ar and horn- 

 blende ; and when examined under a high power, the quartz is seen to 

 contain fluid cavities. The next was a felsite of sp. gr. 2 • 64. Under 

 the microscope this was found to consist of a colourless and generally 

 amorphous matrix enclosing a few dodecahedral crystals of quartz and 

 some small crystals of felspar. Other portions of the matrix appear 

 to be indistinctly crystalline, and to enclose a few laminae of a greenish 

 mineral, probably chlorite. The next was a colmnnous slate of sp. gr. 

 2*66. A section of this slate, made parallel to one of its lines of 

 cleavage, when examined under the microscope, was found to consist 

 of an amorphous matrix through which is somewhat thickly disse- 

 minated a flocculence of a dirty greenish colour, perhaps due to the 

 presence of minute quantities of chlorite. A few well-defined quartz- 

 crystals were also api^arent. The last was a metamorphosed slate of 

 sp. gr. 2*65. A section prepared from a band, apj)arently of highly 

 metamorphosed slate, lying to the east of the foregoing, in which the 

 cleavage-planes had to a great extent become obliterated, was found 

 under the microscope to be chiefly composed of felsj)athic-looking 

 crystals crossing each other in all directions, with here and there 

 some minute scales of chlorite. These crystals, which readily depo- 

 larize polarized light, are nearly transparent ; but the small amount 

 of potassa, soda, and lime present in the rock, as shown by analysis, 

 renders it improbable that so large a proportion of it can consist of 

 any variety of felsjiar. 



