Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/454

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Dr. Mac Culloch on the Geology of

of discrimination will otherwise be sacrificed to the maintenance of an hypothesis, and the student whom it is a duty incumbent on our Society to assist, will learn to adopt that hasty and slovenly nomenclature, which is destructive of correct description, and scarcely less inimical to accurate observation.

Having said thus much on the very ill apprehended and often ill applied term graywacke, I shall be pardoned for suggesting the propriety of limiting it by a certain fixed definition. Different observers have classed under it, substances the most discordant, looking either to their general geological hypothesis, or finding it a convenient repository of rocks for which no other name was at hand. Thus it has become a chaos of ill associated substances. Because Cumberland and Wales are supposed countries of transition, almost every rock found in those districts has been occasionally called graywacke, and thus we have had breccias of all possible modifications, sandstones, and clay slates, confounded with genuine graywacke under one common designation.

The definition of Werner appears precise, and I believe I do not misapprehend it, when I state that its essential part is to possess clay slate as the cement of certain mechanically altered grains or fragments of different rocks. These may vary materially in size, and thus form the two leading varieties of fine and coarse graywacke, and if they also possess a fissile structure, they will then constitute fine and coarse graywacke slate. It is true that in the definition of Werner, as given us by Jameson, the grains are stated to be quartz, indurated clay slate, and flinty slate, but since felspar and fragments of other rocks do occasionally occur in the best characterized graywacke, it would probably be desirable to extend this part of the character so far as to include all grains and fragments, of whatever nature they may be, and to consider the cementing