Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/276

 of those laminated limestones which contain alternate layers of micaceous schistus or clay slate.

There is yet one other disposition of these two classes of rock; this consists in minute points or fragments, if they may be so called, of the same siliceous matter, inhering in the limestone, and which from their smallness are scarcely to be detected, unless where from having been exposed to the action of water they are found to give a rough and echinated surface to the calcareous rock. This appearance is very widely diffused through the whole extent of the body of limestone hereafter to be described, as forming a great portion of this district, wherever it is found in the vicinity of the great mass of granite.

The singularity of these appearances renders it proper to dwell a little on them, and to enquire into their connections and probable origin. Saussure indeed has mentioned a transition from granite to limestone, and, as there are some situations in Glen Tilt where the limestone in contact with granite becomes so siliceous and indurated as by degrees entirely to lose its mineralogical, and pretty nearly its chemical character, so it is possible that he may have met with some similar fact, although no very accurate notion can be derived from his account of it and consequently no assistance obtained from his observation.

As I shall have occasion to enter fully hereafter into the mineralogical description of this and of the other rocks which occur in Glen Tilt, and shall then vindicate the term granite, which as a general term I have applied to it, I think it only necessary to say at present that the mass of rock above described as traversing the stratified rocks, is a portion of a more continuous one which may be traced to the hills constituting the right hand or northern boundary