ISLE OF MAN. GEOLOGY. SCHIST. 543 



appearance very conspicuous. There can be no doubt of 

 the nature of the rocks engaged in these alternations, as 

 one of the beds of slate contains the black chalk, or 

 drawing slate, which was mentioned in the general 

 description. 



I need take no notice of the flexion and apparent com- 

 pression of the clay slate in this instance, as I can add 

 nothing to what I have on former occasions remarked on 

 this subject. 



The next instance is to be seen in the vicinity of 

 Douglas. Here, the clay slate is also the predominant 

 substance, and is disposed in laminae at a high angle, 

 often nearly vertical, and of great extent. These, from 

 their magnitude, convey the idea of beds. In examining 

 them more narrowly, a belt of rock, resembling at first 

 sight a trap vein, is seen crossing them in a nearly 

 horizontal direction. On investigation, this is found to 

 be a mass of graywacke, not divided by any fissure or 

 discontinuity from the slate, which lies above and below 

 it ; although when broken, the one rock readily separates 

 from the other. The fissility of the slaty rock is in 

 this place very remarkable, since it is easily split into 

 large tables, and also exfoliates naturally in flakes of 

 extreme thinness. But this fissile quality is interrupted 

 at the place where the graywacke appears, being again 

 renewed as before, above it; the breadth of the gray- 

 wacke zone not exceeding two or three feet. The fracture 

 of this latter is angular and irregular, without the slightest 

 appearance of a slaty structure, and its texture is rather 

 fine-grained. The subjoined sketch will afford a clearer 

 elucidation of this circumstance.* 



It will be rendered more intelligible by calling to mind 

 a similar circumstance occurring in Isla, and already 

 described, as the one will serve to illustrate the other. 

 In the present example, the fissility is indeed suspended 



* Plate XXVIII. fig. 1. 



