486 Dr. Mac Culloch on Ouart% Rock, 



all the varieties which consist of mere quartz, does not exclude 

 those which contain a mixture of other ingredients, more than the 

 terms sandstone or limestone exclude the various bodies which so 

 often are mixed with, or constitute an integrant part of them. 

 When more particular description is required, a term so general 

 admits of being easily combined or modified. 



We have now to enquire into the rank which this rock bears in 

 the generally received arrangement, and into its claims on a place 

 in either of the classes of chemically crystallized or mechanically 

 deposited rocks, or of that which is conceived to be a mixture of 

 both ; to use language more hypothetical, we must try to assign it 

 a situation in the primitive, transition, or floetz divisions. It 

 appears that it is found, as at Tyndrum and Loch Leven, alternating 

 w^ith micaceous schist, and in many other places with what is called 

 primitive clay slate. Thus its claim to a rank among the first 

 division of rocks is established. Its connection with graywacke in 

 Jura, equally establishes its claim to a place among those rocks 

 which are assigned to the second division. It is thus, like clay 

 slate, a member of both these formations. I have already shown, 

 that at Balahulish, as well as in Jura and in Assynt, the quartz 

 rock contains mechanical deposits, from which it must follow that 

 the existence of a mechanical deposit is not a decisive character for 

 the rocks of transition, since a rock of which one of the charac- 

 teristic circumstances is mechanical arrangement, is found to exist 

 among the rocks called primitive. As I have also shown in 

 describing the country about Aberfoyle, that a gradation from 

 mica slate to graywacke takes place by insensible degrees, I think 

 we may conclude that no valid distinction, nor any constancy of 

 character, such as ought to constitute a class, is to be found in the 

 rocks of transitioji ; and that it would be preferable to return to the 



