NATURE AND ORIGIN OF MICA SCHIST. 



375 



consequence of the irregular size and arrangement of the pieces of 

 quartz ; and the undulations thus occasioned on the micaceous surfaces 

 are often further mo- 

 dified by interspersed 

 garnets, the growth 

 of which appears to 

 have pushed aside the 

 other ingredients. Be- 

 sides this minute in- 

 equality, the laminae 

 of mica slate are liable 

 to the same contor- 

 tions and curvatures 

 those of gneiss ; 



as 



-Mija Schist at Loch Lomond. 



Fig. 73- 



the same difficulty often occurs in tracing its beds ; similar and very 

 numerous veins of quartz traverse and mingle with its layers ; and 

 when in contact with granite it is locally penetrated by similar granite 

 veins. Small cavities lined with crystals appear among the most con- 

 torted parts. 



Mineral Constituents of Mica Schist. The kind of mica in 

 mica schist varies with the locality. In the St. Gothard the soda 

 mica Paragonite is found. In some localities the yellowish- white 

 potash mica is rich in water, and forms the species Damourite. The 

 colours of the mica vary, but dark magnesia mica is most common. 

 This mineral determines the colour of the schist, which is grey, or 

 greenish-grey, or yellow-grey, or may be brownish black. 



The quartz occurs in grains, scattered between parallel layers of 

 mica scales. As the quantity of quartz increases, the grains become 

 large, flattened lenticular plates, among which films of mica are diffused. 

 Occasionally the quartz becomes so abundant as to be only separated 

 into layers by thin films of finely-divided mica, and such varieties 

 make a transition to quartzite. The varieties which are poorest in 

 quartz always have*sinall grains of quartz enveloped in the laminae of 

 mica. The varieties of texture are similar to those of gneiss ; but the 

 crumpled wavy structure is one of its most typical modifications. 



Ripple-Drift in Mica Schist. Dr. Sorby has described the struc- 

 ture of mica schist, in which he recognises the " ripple-drift " seen in 

 sandstones. This drift structure is by no means rare in the mica 

 schist of the Highlands of Scotland. It may be well studied between 

 Aberdeen and Stonehaven, and between Arrochar and Tarbat. In 

 many specimens of typical mica schist the original grains of sand may 

 be recognised under the microscope, because the original quartz is full 

 of fluid cavities, and is surrounded by the clear quartz of the schist, 

 which may similarly enclose grains of felspar sand. 1 



Flexures of Mica Schist. The accompanying sketches (figs. 73, 



75) were taken from the mica schist near the anticlinal axis of 



these beds which crosses the upper part of Loch Lomond. On a 



f/rcat scale, the laminations of gneiss and mica schist are suffi- 



1 Q. J. G. S., vol. six. p. 401. 



