Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/317

 has not hitherto been found in Scotland, and as indeed it seems to have been as yet seen in only one or two situations besides the place whence its name is derived, it will not be superfluous to describe the specimens and the substance in which they are imbedded. The base of the porphyry is a brownish hornstone or compact felspar, containing numerous crystals of reddish-yellow felspar and a few grains of quartz, some of which appear to have the primitive form of this substance. Together with these, numerous crystals of pinite are imbedded in the stone, varying in diameter from one tenth to one sixth of an inch, sometimes of similar length, and at others mere scales. Brongniart describes it as having been found in porphyries, and I may add that the specimens which I procured differ in no respect from the foreign specimens with which I have compared them.

A similar porphyry, I may here say, is to be observed in Glen Shee. Oxide of titanium in a pulverulent or scaly and investing form, is also to be seen in the rifts of the quartz rock in Ben Gloe. The same disposition of this mineral occurs in Ben-na-caillich, near Killin, and it must not be confounded with oxide of iron, the more common metallic substance in rifted quartz. Among the loose stones on the skirts of Ben Gloe I also found a singular substance. It is a laminated schist of which the basis is clay slate, occasionally mixed with hornblende and with quartz, and containing small lamellar and conchoidal scales of a crystalline white carbonat of lime. I know not how to describe this rock so well as by saying that the calcareous bodies are precisely like fragments of shells, and that the whole on a first view would pass for a shale containing shells; if found in a secondary country it