LITHOLOGY. 237 
that something more than the mere cementation of the angular grains 
that constitute clay has taken place; and the rocks are therefore clas- 
sified as half fragmental. The amount of this crystalline product varies ; 
and when it becomes prominent, and the fragmental character is no longer 
apparent, the rock becomes argillitic mica schist. The laminated struct- 
ure has been induced in slates by pressure, and it may coincide with the 
bedding, or it may not, according to the direction in which the pressure 
was applied. This lamination is usually more regular, and is confined 
more strictly to a level plane, than is that of the other schists, and the 
ready separability of the laminz produces what is termed the slaty 
cleavage. Since this cleavage is always in a plane at right angles to 
the pressure, the direction of the pressure which produced it may be 
assumed from its plane. Therefore those argillaceous rocks in which 
the recrystallized elements are not so prominent as to impress their char- 
acter upon the rock, and which possess a slaty cleavage, are considered 
as clay slates. 
The so-called roofing slates are typical of this class, and of them a 
specimen from Littleton has been selected and examined. It is nearly 
black; and only with difficulty can a section be made sufficiently thin to 
allow of satisfactory study. It is then seen to consist of a mixture of 
quartz and feldspar in fragments as fine as dust. The mixture is rendered 
black by the inclusion of a considerable quantity of some amorphous 
coaly matters; but all through this formless mass of materials little 
needles or fibres are seen, which are brightly colored in polarized light, 
and which constitute the crystalline portion of the rock. As to the 
nature of these minute crystallites that are always found in clay slates, 
no certain conclusion has been reached. Mr. Zirkel thinks that they may 
be hornblende, and says that every careful investigation of their mode of 
arrangement points to the fact that they were formed previously to the 
consolidation of the rock. As no softening of the rock by the processes 
that have operated in the crystallization of the schists is evinced in the 
structure of the slates, this must be true; but that the needles in our 
slates are of hornblende, I doubt, for where good opportunity for examina- 
tion is found, their optical behavior is not that of inclined crystals, but 
they act much more like the fibres of mica, which in the argillitic schists 
become so much more prominent and easy to recognize. 
