2l8 MINERALOGY AND LITHOLOGY, 



ARGILLITIC MICA SCHIST. 



In mica schist, by insensible degrees tlie scales of mica lose their in- 

 dividual character, till at last nothing that can be distinctly recognized 

 as a scale or crystal can be seen. All the constituents are very fine, 

 and the rock becomes argillitic mica schist. The rocks of this class, 

 however, possess a glistening lustre which is suggestive of mica, and 

 which is due to its presence, as the microscope indicates. With a high 

 magnifying power, they are seen to be composed of an entirely crystal- 

 line mixture of quartz, feldspar, mica, and chlorite, with various accesso- 

 ries. They are distinguished from real argillites and slates by this crys- 

 talline type, but the metamorphism has been of such a gentle nature that 

 it has left the ingredients in a fine condition, and has not entirely re- 

 moved the argillitic character. The condition of the ingredients and 

 the presence of accessories create many variations in this rock. When 

 much chlorite is present, the schists are greenish in color. Excessive 

 fineness and a preponderance of the micaceous ingredient give a soapy 

 feel to the rock. Contortion of the laminae produces wavy, woody, and 

 other imitative structures. But under the microscope, these rocks are 

 all essentially alike, and are characterized by their very fine yet entirely 

 crystalline type and micaceous appearance. Here are included the rocks 

 called phyllite, and the larger part of those called hydro-mica slate. The 

 rocks called by the Germans old clay slates (urthonschiefer), as distin- 

 guished from the newer slates, are of this class, and many local names 

 have been given to varieties. Just as intermediate kinds bind this rock 

 to mica schist, so other varieties, which are not so entirely crystalline, 

 bind it to the clay slates. In general, however, it is very well character- 

 ized, and quite distinct from these related rocks. 



Argillitic mica schist is very abundant in New Hampshire, particu- 

 larly in the Connecticut valley. For special examination, I have selected 

 a specimen from that region, which is as typical as possible of the whole 

 group. It is what is called by Prof. Hitchcock the Lyman schist, which 

 by itself composes the larger part of a formation. The specimen was 

 collected at Woodvillc. It is gray in color, very schistose, but not at 

 all friable. It is in appearance perfectly homogeneous, and no constit- 

 uent can be detected with the eye or pocket lens. It is somewhat glis- 



