224 MINERALOGY AND LITHOLOGY. 



Orthoclase often appears in these rocks, but only as an unessential 

 accessory, though sometimes it is macroscopically visible. Plagioclase 

 is not at all rare. Apatite is found in a section of a Portsmouth quartz 

 schist. Pyrites is a very common ingredient. It occurs in both micro- 

 scopic and macroscopic crystals, which though so small are often very 

 well formed. 



A quartz schist from Connecticut lake is very fine in its texture, and 

 contains, as accessories, calcite, plagioclase, biotite, magnetite, and chlo- 

 rite. Calcite is not rare in these siliceous schists. Its common pres- 

 ence in such acidic rocks points toward the gentle action of the forces 

 that have effected their recrystallization. This is apparent in all the 

 rocks of the region where these schists are most common. 



GREENSTONES. 



Under this head it is proposed to describe those interstratified rocks 

 which are so prominent in the valley of the Connecticut, which are inti- 

 mately associated together, and which, in common, possess a more or less 

 green color that is induced by the presence in them of hornblende or 

 chlorite. No especial significance is intended to be attached to the 

 word greenstone, though efforts have been made to dignify it with a 

 meaning. I use the word simply to connect together for discussion cer- 

 tain rocks, which in any lithological classification would be widely sepa- 

 rated. As warrant for their union under this term, there is their close 

 geological association, and the precedent established by all those who 

 have written upon the geology of the valley region, who have so con- 

 stantly spoken of greenstone that all will understand what rocks are here 

 referred to. 



All along the valley of the Connecticut, from its source to its mouth, 

 there are large accumulations of green rocks, and so characteristic are 

 they of the region, that the area has always been colored green on geo- 

 logical maps. These rocks are of several kinds, and are interstratified 

 with one another, with argillitic schists and with other related rocks. 

 One kind is the light green chlorite schist, which the microscope shows 

 is essentially composed of minute grains of quartz with chlorite. Another 

 is the amphibolitc, which is composed of amphibolc mixed with more or 



