Page:Quarterly Journal of the Geological Society of London, vol. 29.djvu/175

1873.] to the patches in Ross ; and Prof. Ramsay informs me that, when he examined the Brora district some years ago, he was led to adopt the same views with regard to it.

I have now to point out two very remarkable and interesting phenomena, which, while they are on the one hand altogether anomalous and inexplicable, except on the hypothesis that the two series of rocks have acquired their present relations through the agency of great faults, are, on the other hand, seen to be in most complete harmony with the inferences to which all the other facts have led us.

It will be shown hereafter that the Upper Oolite beds in portions of this district are almost wholly made up of derived blocks. These blocks so precisely agree in mineral character with the Caithness Flagstones and associated beds of the Middle Old Red Sandstone, that Sir Roderick Murchison was evidently strongly inclined to refer them to this source. One fact, however, appeared to offer insuperable difficulties to accepting such a conclusion. The rocks which now appear in closest proximity to the Secondary strata in question are the Ord Granite, and the conglomerates and sandstones of the Lower Old Red, while the Middle Old Red Sandstones were then known only at a considerable distance. Sir Roderick pointed out the remarkable fact, which subsequent observations have completely confirmed, of the total absence of fragments of those well-marked and most easily recognizable rocks (the Ord Granite and the Old Red Conglomerate) in the "brecciated beds".

So fatal did Sir Roderick consider this fact to the hypothesis that the blocks in question were derived from the Palaeozoic strata, that he found himself compelled to abandon it; but it is with evident doubt and reluctance that he resigns the theory in question in favour of another.

As I shall have to show more fully hereafter, the fact that these blocks in the "brecciated beds" are derived, and that they are of Middle Old Red Sandstone age, is put out of all question by the discovery in them of the remains of the characteristic fishes. The difficulty pointed out by Sir Roderick Murchison reappears therefore with its full force.

If, however, we admit that the present position and relations of the Primary and Secondary strata are due to a great fault, this startling difficulty at once disappears; for the thick series of the Middle Old Red Sandstone, so magnificently developed in Caithness, where it has escaped denudation, might then have formed the lands bounding the Oolitic sea, while the granite of the Ord and the conglomerates and sandstones of the Lower Old Red were buried below thousands of feet of newer rocks.

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