l 



PYEOCLASTICS OF HEMLOCK FOKMATION. 145 



illustrates the appearance of the thin section of such consolidated sand, 

 which possesses in no place the structure of an igneous rock, and which, 

 moreover, grades into a finely banded rock composed of minute needles of 

 hornblende, chlorite, and grains of epidote, lying- in a clear minutely crys- 

 talline groundmass of quartz or of feldspar, or possibly of both. 



The finer material cementing the recognizable fragments is the same as 

 it is in the tuffs. The large bowlders and pebbles lie in a matrix of smaller 

 pebbles, and these in turn lie close together in a paste which has been com- 

 pletely altered, and does not in all cases show clastic characters. The 

 cement is composed of hornblende, chlorite, sericite, epidote, feldspar, and 

 quartz, and in one case large porphyritic rhombs of a ferruginous cai'bonate 

 are scattered through the finer-grained material of the cement. 



In the cement of the tuffs hornblende is present in large quantity and 

 feldspar is not so abitndant. In the cement of the conglomerates feldspar 

 seems to be rather plentiful, hornblende is present in a comparatively small 

 quantity, and chlorite is more abundant, thus reversing the order of these 

 minerals in the conglomerates. 



SCHISTOSE PYROCLASTICS. 



At various places in the Hemlock formation there occur clastic rocks 

 which have become schistose. Two isolated exposru'es of pyroclastics are 

 known whose characteristics have been so changed that, while recognizable 

 as elastics, it is impossible to say whether they belong to the eolian or 

 the water-deposited class. Upon weathered surface the rock is covered 

 with brownish ochre, and on fresh fracture it is dark green and very schis- 

 tose. Neither in exposures nor in hand specimens does it give any indication 

 of its origin. 



In thin section, however, one may see macroscopically the fragmental 

 characters. The fragments are elongated and rounded. The amygdaloidal 

 texture is also seen, showing the volcanic nature of the fragments, though 

 the majority of the fragments are dense. Under the microscope the frag- 

 mental natm'e of the rocks as a whole and the volcanic character of the 

 fragments forming it are still more clearly seen. In the centers the frag- 

 ments are seen to be composed of chlorite flakes in siTch great quantity as 

 partly to conceal the character of the clear white cement, which is supposed 

 to consist for the most part of quartz, though lath-shaped areas with poly- 

 MON xxxvi 10 



