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 f ragmental. 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 laminas 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. 



