456 SHALE. 



They are found in the lowest red sandstone, 

 under a great variety of character; and of this, 

 Arran presents a very striking example. Very 

 commonly also, in this case, they resemble the 

 graywacke of the primary class so strongly, that 

 they cannot be distinguished by their mineral cha- 

 racters ; a circumstance already mentioned in 

 treating of that rock. In some instances, as in 

 the Orkney Islands and on the north-west coast of 

 Scotland, these beds occupy a space so consider- 

 able among the rocks with which they are associ- 

 ated, as to be a source of doubt to the geologist ; 

 since, under peculiar circumstances of exposure, 

 and of limited access to the series, it may thus be 

 impossible to distinguish them from the primary 

 graywacke. In these cases, in fact, there some- 

 times appears to be an actual transition between 

 the primary and secondary classes ; the appear- 

 ance of such a transition being rendered more 

 perfect by the conformable order of the two. 



Shale also forms a member of the various 

 sandstones which lie above the lowest, or red 

 sandstone ; occurring through the greater num- 

 ber of those with which we are acquainted, and, 



