Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/56

 the remainder in an unconformable position to it. But it must rather be considered as the indication of an internal structure, of which however all marks disappear when a fracture is made, a case analogous to what occurs in basalt and many other rocks, of which the internal structure is so often detected by the changes which they undergo on exposure to the weather. A similar case, which I have observed in the argillaceous schist of Isla, serves to confirm this explanation. The appearance in this instance is so precisely like, that the same drawing will serve to represent both, and the nature of the cause is amply confirmed in this, that while the beds of the schist are apparently divided in the direction marked by the horizontal lines, they are fissile only in that which is marked by the oblique ones. It hence also follows, that if the fissile property of clay slate is the result of some internal arrangement analogous to crystallization, we are equally entitled to attribute the structure of this sandstone to the same cause. The inclination of the oblique laminæ upon the horizontal ones varies from ten to thirty degrees; but it is regularly in the same direction, the dip, if it may be so called, being to the south. This appearance is neither rare nor dubious, it is extremely well marked, and predominates throughout the whole range. The measurement of the parts having been mislaid, I speak from recollection when I say that the intervals between the horizontal lines vary from one foot to a foot and a half There is no difference in the quality of the two sets of laminæ, both are of white sand. stone, generally more or less calcareous.

To enter more minutely into the composition of these beds, let me now remark that the sandstone often acquires the aspect of some of the most compact and crystalline varieties of quartz rock, while in other cases it has the lax texture of an ordinary freestone. In some places it is calcareous, and the calcareous matter varies so