Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/274

From Wikisource
Jump to navigation Jump to search
This page has been validated.
Dr. Mac Culloch on the Vitrified Forts of Scotland.

ments of trap cemented by a whitish substance, which proves to be the hard variety of calcareous spar, mixed with a sand of trap. This trap sand is generally of a dark purple colour, resembling many of the imbedded pebbles. Although this sand is the predominant ingredient in the paste, there are also found in it grains of quartz, minute zeolites, garnets, crystals of calcareous spar, and here and there prehnite, diallage, and chlorite slate, as far as it is possible to speak decidedly of objects so very minute. The spar which cements this sand into a common paste surrounds every grain so as to form them into a perfect breccia, and enable the whole to break with the splintery fracture above noticed, instead of a granular one. Here and there the paste occupies large interstices which have been formed by the approximation of two convex surfaces of considerable extent, and from these it may be traced insinuating itself through all the grains of the mass. It is evident that the calcareous spar has been introduced while in a state of fluidity among the sand and gravel, as the larger pieces of paste may be observed to envelope the grains of trap. Generally therefore we may consider the pudding-stone of Lorn as a congeries of trap sand and trap pebbles, cemented by calcareous spar, a rock often designated by the improper name of trap tufo. This however is not the place to enquire into the means by which the mass was consolidated. That it is a case of an agglutinated rock differing greatly from the ordinary sandstone breccia, or the ferruginous and argillaceous pudding-stone, is very apparent. It resembles them indeed only superficially and in its mechanical texture, and it will be worthy the labour of geologists to direct their attention to the pudding-stones of this coast with more care than they have hitherto done. The other rocks are too well known to need any description.