SoLLAs — On a Fragment of Garnet Sornfels. 53 



formula requires one atom more of hydrogen than was found 

 on analysis, but probably this is due to the presence of some 

 molecules, in which one of the rays has the formula — 



0— Mn 



= Si^ \. 



^0— Fe/ 



In the rock slice under the microscope transverse sections 

 of this mica appear as parallel-sided areas, with more or less 

 jagged ends, the usual basal cleavage is well developed, but 

 frequently much curved or distorted : the colour is bluish-green : 

 the mineral is evidently dichroic. When the cleavage planes lie 

 at right angles to the long axis of the polariser the colour is sage 

 green ; when parallel with it scarcely any colour is perceptible ; 

 pleochroic aureoles surround included zircons, just as in the 

 biotite of the more normal mica schists of the district. The angle 

 of extinction cannot be determined with any approach to cer- 

 tainty — partly owing to the curvature of the cleavage planes, and 

 partly because it does not appear to be uniform ; in many cases it 

 is 0°, in others 7°, 9°, or even 14°, as measured with the trace of 

 the cleavage planes. The flakes obtained by separation are rarely 

 bounded by cleavage planes at the edge ; usually their outline is 

 altogether irregular. As they lie flat on the principal cleavage, 

 many remain extinct during a complete revolution between 

 crossed nicols, but some do not, patches being illuminated, while 

 the rest of the flake remains dark. These optical irregularities 

 are, doubtless, connected with the deformation of the crystals, 

 resulting from pressure and shearing. 



A good deal of muscovite is present, and it also presents 

 curved cleavage planes and undulose extinction. 



The larger part of the rock consists of a mosaic of irregular 

 angular grains, more or less colourless and transparent, lying 

 with their longer axes in the plane of schistosity. Some of these 

 are quartz, with very marked undulose extinction ; others some 

 kind of felspar, in which repeated twinning on the albite, and 

 sometimes also on the pericline plane, can be frequently detected. 

 Though on the whole remarkably clear and colourless, yet finely 

 granular decomposition products are usually present, and produce 

 a more or less yellowish tinge. 



