PYROCLASTICS OF HEMLOCK F0RMAT10]S;. 147 



PI. XXX.) These specimens have the characters of the porphyritic schis- 

 tose lavas described above, but show clearl}' that the^s' have lieeii derived 

 from igneous clastic rocks. 



Other schistose elastics occur in larg-e quantities in sec. 24, T. 46 N., 

 E,. 33 W. They are penetrated by a boss of coarse poikilitic dolerite 

 which may have aided in rendering them schistose, though their schistosity 

 agrees with the general strike of that of the rest of the district, and is 

 probably chiefly due to the general folding. 



Macroscopically their clastic structure mav be clearly seen. The peb- 

 bles are dense greenish gray in color and oval in outline. The matrix is. a 

 much darker green. The schistosity of the rock is marked. The pebbles 

 are uniformly elongated, and they have the appearance of having been 

 mashed. The schistosity agrees in direction with the elongation of the 

 pebbles. 



The microscope shows the pebbles to be basaltic, with a type of struc- 

 ture intermediate between the navitic and intersertal structures, and another 

 with approach to the trachytic structure. Considerable brown mica is 

 present in both kinds, and occurs in flakes which are probably secondary, 

 though the pebbles show few traces of alteration. The matrix consists 

 essentially of actinolite in rather coarse needles, large grains of fresh 

 magnetite, but ^'erj- little mica, and that such as is seen in the pebbles, 

 all l\'ing in a cement of quartz and calcite. The passage between thf 

 cement and the pebbles is a more or less gradual one, there being a change 

 as we pass from the center of the pebble, where isolated actinolite needles 

 and epidote grains occur, toward the edge, where these minerals increase in 

 amount until between them here and there are twinned feldspars. In the 

 matrix proper the quartz is the predominant white silicate, though here and 

 there limpid feldspar is also seen. The pebbles are gradually being eaten 

 up, so to speak, by the actinolite, and we can imagine the final result to be 

 an actinolite-schist showing no clastic structure, and giving absolutely no 

 indication as to the rock from which it originated. 



There is no microscopical evidence of mashing in the minerals, and 

 since this is absent from the quartz in the cement, I conclude that no original 

 clastic cement is now present, and that the quartz is a secondary crystalli- 

 zation product derived from infiltrated material and from material obtained 

 from the adjacent pebbles. Whether the rock is an eolian deposit or a 



