Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/466

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Dr. Mac Culloch on Quartz Rock.

is the perfect crystalline texture of quartz any proof of a primitive formation; since the strata of Kirkaldy, which belong to the flœtz and coal formation, contain beds of highly crystalline and translucent quartz, alternating with coal and organic limestones.

The mode in which the felspar exists in this rock proves also its mechanical construction, and shews that it has very probably been derived from the wearing and deposition of antient granites, by whatever means we may attempt to account for its present highly compacted and often crystalline structure.

Having thus, as I trust, clearly proved that the rock of Jura is a recomposed rock, as far as regards its structure, and that it cannot from its compound nature be properly entitled to the name of a granular quartz, it is our business to consider its geological appearances, and the probable nature of the process by which it acquired its present disposition.

Before examining this question, it is requisite to premise an observation of professor Jameson, whose accuracy is not doubted. He remarks that the “Quartz rock rises at an angle of 45 degrees from under the micaceous schistus.” Here then we have a rock formed of a mixed mechanical deposit, lying under a rock which is held to be primitive, and the third in succession from the first and fundamental of all rocks, granite. I did not observe this fact, but have perfect reliance on his observations, because they coincide with what I have myself observed in other places. Either then this mechanical deposit must be considered as a primitive rock, or micaceous schistus is a rock of more recent formation than it has been generally esteemed.

This latter supposition may perhaps admit of further proof, but I cannot enter into it in this place.

Leaving this additional difficulty out of discussion at present, it is