360

THE FOSSILIFEROUS DEPOSITS

occupy nearly ten thousand square miles of its area,—werethen but feebly developed, compared with its many-colouredporphyries, its granites, and its quartz-rocks. In the Forfar-shire Conglomerate, the prevailing rocks are hard porphyries,of an infinite variety of hue, and indistinguishable in theircomposition from the porphyries of Ben Nevis and Glencoe;in the Conglomerate of Cromarty and Boss, a decaying gra-nite, red like that of Peterhead, but as finely grained as thatof Aberdeen, blent with red quartz-rock and red graniticgneiss, is the prevailing stone; in that of Orkney , as exhibitedin the neighbourhood of Stromness , the prevailing rock isalso a red granite, somewhat larger in its grain, and moredurable, than the Cromarty one. The stone which composesmany of these inclosed pebbles can no longer be found insitu; and a good representative collection of at least theclasses of rocks which they exemplify would serve to showthe nature of the framework of that ancient unknown landto whose existence the Great Conglomerate bears evidence;and which—as over many thousand square miles the pebblespresent the worn and rolled character—must have been ex-posed, zone after zone, during a protracted period of gradualdepression, to the incessant wear of the ocean. The Conglo-merate seems to have been exposed in an after period to in-tense heat. We find many of its hardest pebbles bent andindented, as if they had been reduced to the consistency ofdough, or distorted by miniature faults, which scored theirlines of fracture with the ordinary slicken-sided markings,when they were in a state viscid enough to re-unite. Thefact would have been deemed a very great one during theheat of the controversy waged in this city between the anta-gonist schools of Hutton and Werner ; but it is not less in-teresting now, when it can be looked at more quietly; andso I have given to a series of the pebbles which illustrate ita place in my collection.