Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/471

From Wikisource
Jump to navigation Jump to search
This page has been validated.
461
Dr. Mac Culloch on Quartz Rock.

stance highly instructive with regard to the composition of many other rocks. Of these natural analyses, a very common one is the appearance of laminæ of red and white matter, alternating either in flat plates, or in that peculiar undulating form so well known in the flœtz sandstones, and marking the action of water on loose sand. An occurrence of equal importance and greater singularity is that of imbedded cylindrical bodies which they occasionally exhibit. I may previously remark that they have a frequent tendency to break into rectangular solid masses, similar to those which occur in many flœtz sandstones. In these fragments the weathered surfaces present on the upper part, or that which forms the plane of the stratification, a number of circular protuberant spots, apparently arising from the circumstance of their hardness being greater than that of the general mass. The lateral plane of the same fragments exhibits on the other hand a similar number of corresponding cylinders, of a hardness in the same way superior to that of the surrounding parts. If I might venture on a comparison as vulgar as it is explanatory of this appearance, I would compare it to the two sections of a piece of larded meat. I may further add, that in these cases the cylindrical bodies are of a much whiter colour, as well as of a more compact texture than the rest of the stone; and that on breaking the stone to examine further into this structure, the whole disappears, and an uniformity of texture is exhibited throughout.

This peculiar appearance is familiar to all those who are conversant in the varieties of flœtz sandstone, and the superior hardness of the cylindrical or vermicular bodies in the sandstones of this class is equally notorious, as they often continue to project for the space of even half an inch beyond the decomposing rock, appearing as if nails had been driven into it. The coast of Fife about Burnt Island affords excellent examples of this fact. I know not that any attempt