Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/420

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Dr. Mac Culloch on the Geology of

having been subsequently filled up by red or white sparry matter, a beautiful variety of marble is produced, which is now wrought for the purposes of ornament. Its cavities also occasionally contain large and perfect crystals of that form of calcareous spar called equiaxe. Its surface is in most parts covered with a loose calcareous tufo, which in some places being solidified by an infiltration of calcareous matter, forms beds of solid breccia. It contains grains of sand dispersed throughout, and for this reason, commonly gives fire with steel, a circumstance of very frequent occurrence in all stratified limestones, whether referred to the rocks called flœtz, or transition, and generally most conspicuous in those which contain animal remains in abundance. Such are the circumstances of chief importance in the character and position of this limestone.

I shall now, perhaps, be expected to assign a place to this rock in the usual division of primary, transition, and flœtz, distinctions which I am inclined to think are more easily made in the closet than in the field. In the present state of geological science, it would appear a safer practice in this case, as in many others, to describe that which actually exists, without the use of hypothetical terms, which only serve to perplex the observer, and to mislead the student, who either boldly pronounces on the character which suits his particular creed, or modestly supposes himself incapable of sound observation, because he is unable to see that which is not visible. If the quartz rock of this tract be a primitive rock, then the limestone is necessarily a primitive rock also, notwithstanding its bituminous nature, and accurately stratified structure, since it alternates with it. But, if this rock is not a primitive, but a transition rock, then the limestone also must be removed from the first class of rocks to the second.

In examining its right to a place among the transition rocks, we must compare its characters with those which have been assigned as