THE ORIGIN AND DESCENT OF ROCKS. 



419 



express the leading character of the rock without implying a knowledge 

 of the precise mineral composition. A convenient term of this kind is 

 greenstone, which merely indicates that the ferromagnesian minerals 

 are prominent and usually give a greenish or dark cast to the rock. The 



l#^ 



Fig. 347. — Brecciated limestone, Calciferous formation. 



Highgate Falls, Vt. 



One mile south of 



greenstones embrace the diorites, dolerites, some of the gabbros and 

 the basalts, and may even extend to the peridotites and some of the more 

 hornblendic of the granitoid rocks. Another convenient name is trap. 

 which may be used for any dark, heavy igneous rock. The name (from 

 ^rappe, stairs) refers to the step-like arrangement which the edges of 

 the superimposed sheets of lava often take, especially when the lava 

 is of the free-flowing, basaltic kind. 



