Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/145

 porphyries that occur in Glenco and correspond with those which are every where found in veins among the older rocks, being entirely absent in those which lie above the red sandstone. Are we to attribute these diversities to a different era of formation? Unfortunately our knowledge of these rocks is as yet so limited that this question cannot be answered; but the few remarks which precede may for the present remain as slender contributions towards their history, independently of the local interest they may possess in illustrating the description of Cruachan.

The next rock of which it is necessary to take a somewhat more extended view, for the purpose of illustrating the structure of this mountain, is the red sandstone, together with the white calcareous sandstone which lies above it. As the former is of frequent occurrence while the latter is only occasionally present, and as there is no difficulty respecting the consecutive position of these rocks, I shall neglect it in the short notice here to be given.

It has been shewn that these sandstones occur at the foot of the mountain, occupying a very small space, and that no continuation of them can be traced to the immediate vicinity. But in traversing a larger portion of this tract of Argyleshire a similar phenomenon is found frequently occurring, which, if it diminishes the suprize at first excited by this very limited extent of the secondary strata in the spot above described, adds a much greater interest to the fact, while it naturally leads the geologist to enquire into the circumstances under which so remarkable a dismemberment of these strata has taken place. Their continuity and extent as they occur on the eastern and southern skirts of the highland mountains are too well known to require notice, and they will be found to occupy an extent similarly continuous, as far as geographical circumstances will permit, on the