﻿IOO NEW YORK STATE MUSEUM 



k;u es or needles; so that, in so far as the mineral particles have 

 longer diameters, or scalelike shapes, these develop in the planes 

 at right angles to the direction of compression and give the rock 

 a tendency to split along them. Obviously a better cleavage 

 will usually develop in rocks which consist of more than one 

 mineral than in those composed chiefly of a single one, and in 

 the former case a better cleavage will appear where there is 

 large difference in the characters of the different mineral species 

 than where this difference is small. Thus a quartz-mica rock, or 

 a feldspar-hornblende rock, will be apt to have a much better 

 foliation than a quartz-feldspar rock. 



A rock in which a good foliation cleavage is developed, so that 

 it tends to split rather evenly and readily is said to be schistose, 

 or called a schist. When the foliation is less even, and less 

 ready, gneissoid is the adjective, and gneiss the substantive em- 

 ployed. As a general rule certain sediments, such as shales and 

 impure (or shaly) limestones and sandstones, recrystallize into 

 schists, while pure sandstones and limestones, which recrystallize 

 into pure quartz or pure calcite rocks, and consist chiefly of the 

 one mineral, show little or no foliation. Igneous rocks are 

 usually already crystalline, and in general do not recrystallize 

 with as prominent a foliation as do many of the sediments, hence 

 are more prone to form gneisses than schists. 



Foliation in the Grenville rocks. The pure Grenville quartzites 

 and limestones are now quite massive crystalline rocks with 

 little or no foliation, though there is some development of frac- 

 ture cleavage in the resistant quartzites, which is lacking in the 

 more plastic limestones. Even the quite impure limestones show 

 usually but little foliation. The impure quartzites have de- 

 veloped either pyroxene or mica on recrystallizing, usually the 

 former, and this rock has poor cleavage while the latter become 

 quartz schists. In the mass of Grenville rocks of varying com- 

 position to which the general name of the " schist series " has 

 been applied, foliation cleavage is in general prominent. But 

 even here rocks with considerable development of minerals of 

 the mica type are relatively rare, and since such constitute the 

 most prominently foliated rocks, their rarity militates against the 

 prominence of foliation in the series, the bulk of which would be 

 better classed as gneissoid, rather than as schistose. Some varie- 

 ties of the amphibolites are quite micaceous and hence possess 

 good foliation cleavage. The green schists and ordinary amphi- 

 bolites usually show fair foliation only, and a general assemblage 



