418 SUPERIOR SANDSTONES. 



another known by some other name. But this 

 is the peculiar business of geology. 



The occurrence of coal, as an alternating sub- 

 stance in the lowest red sandstone, is so rare, that 

 no notice was taken of it in the preceding chap- 

 ter ; although it is known so to exist in the island 

 of Arran. But, in the upper sandstones, it fre- 

 quently occurs ; or rather, the beds of coal are 

 almost necessarily accompanied by alternating 

 strata of these rocks. 



Where mica is found in these sandstones, it 

 is often the cause of a laminar tendency, and, as 

 in the red sandstone, flags of considerable value 

 for ceconomical purposes are thus generated. 

 This cannot, however, be considered among the 

 modifications of structure ; but those forms 

 which may fairly be referred to such a cause, are 

 very conspicuous in some varieties*of this rock. 



The first of these deserving of notice, is truly 

 laminar, and independent of the intervention of 

 mica, or of any other substance. The sandstone 

 in which it is found, is composed of quartz alone ; 

 and the tendency to split, is generally only de- 

 tected by the action of the weather. The mode 



