Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/496

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Dr. Mac Culloch on Quartz 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 integrand 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 situation in the primitive, transition, or flœtz divisions. It appears that it is found, as at Tyndrum and Loch Leven, alternating with 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 established 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 mock 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 characteristic 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 transition; and that it would be preferable to return to the