220 MINERALOGY AND LITHOLOGY. 



recrystallized minerals, and do not share at all the fragmental character 

 of the rocks like clay slates, which are composed of the merely cemented 

 particles that constitute clay beds. To these rocks they are, in composi- 

 tion and physical characters, so nearly related that they are appropriately 

 termed argillitic mica schist, and they form the connecting link between 

 mica schist and argillite. 



No analysis can be called typical of such rocks as these, for the pro- 

 portion of silica in them is as variable as it is in beds of clay. With 

 further increase of the micaceous constituent, the rocks become more 

 glistening in lustre and softer to the feel. A schist of this nature at 

 Lyman is commonly called the copper schist, because it contains con- 

 siderable chalcopyrite and bornite. Pyrites is a common constituent of 

 this class of rocks. 



Some of these schists are very dark in color, and thin sections of such 

 are filled with an opaque, apparently amorphous substance. A specimen 

 from Dalton is an example. When a fragment of this rock is heated, 

 the coloring material is destroyed ; and this indicates that the black par- 

 ticles are of some bituminous substance. In these black varieties the 

 glistening appearance is often nearly absent, but, with a pocket lens, in 

 no variety can the mica be more clearly distinguished. 



Like mica schist, this rock species is varied by the abundant presence 

 of certain accessories. As often as mica schist is characterized by anda- 

 lusite crystals, so often is argillitic mica schist by chiastolite; and chiasto- 

 lite seems to be confined to rocks of this class, — for when the micaceous 

 character of the rock is well defined, the macled character of the crys- 

 tals is absent. A macled variety of staurolite characterizes an argil- 

 litic schist at Charlestown, and being an isolated occurrence this stau- 

 rolite scJiist has attracted much attention (see p. no). At this locality 

 the argillitic rock gradually passes into a well defined mica schist, and, 

 as before stated, with this passage the macled structure disappears. 



Some other striking peculiarities in the nature of this rock are seen in 

 certain localities. At East Hanover, for example, some of the schists 

 are mottled by what are apparently pebbles of various sorts and sizes 

 that have been flattened out between the layers. This circumstance was, 

 I think, first noticed by Prof. E. Hitchcock,* in 1833, and was mentioned 



* On the conversion of certain conglomerates into viica schists, etc. E. Hitchcock, Am. Jour. Science, 

 u, vol. 31, p. 372. 



