Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/714



Dr. Sutherland holds that it is obviously a brecciated conglomerate, derived from aqueous deposit, rather than an igneous porphyry, and he urges the following reasons in justification of this view.

In the first place, he has sought for years for the vent through which such a vast mass of erupted rock could have been thrown. Where such a mighty mass is concerned its eruption from beneath could not have been effected without leaving very obvious marks of volcanic disturbance. There are no such indications anywhere. Mr. Bain has suggested the possibility of some such vent having existence about the sources of the Orange river. In that district there is certainly no trace of such rock-convulsion.

Then the abundant ripple -markings, and the gradual transition into fine shales and sandstones of unquestionably aqueous origin, are incompatible with the theory of volcanic or igneous formation ; as is also the rubbed condition of the fragments, and the absence in them of all signs of fire- action or fusion.

And yet, again, there is the transport of the vast ponderous blocks to long distances, and the scorings and groovings of the sandstone upon which the Boulder-clay formation rests, to be accounted for.

On the whole, these circumstances seem to indicate that the constituents of this Boulder-clay have been derived from the superficial denudation of older rocks by aqueous agency, and that it has assumed its existing relations to the other rocks while in the condition of a moist and plastic mass. In all probability the finer shales, containing impress of the ripple-marks, were formed during periods of approximate repose ; and cataclysmic violence and disturbance had ceased altogether when the deposit of such beds as the Maritzburg shales began.

Dr. Sutherland inclines to think that the transport of vast massive blocks of several tons' weight, the scoring of the subjacent surfaces of sandstone, and the simultaneous deposition of minute sand-grains and large boulders in the same matrix, all point to one agency as the only one which can be rationally admitted to account satisfactorily for the presence of this remarkable formation in the situations in which it is found. He believes that the boulder-bearing clay of Natal is of analogous nature to the great Scandinavian drift, to which it is certainly intimately allied in intrinsic mineralogical character ; that it is virtually a vast moraine of olden time ; and that ice, in some form or other, has had to do with its formation, at least so far as the deposition of the imbedded fragments in the amorphous matrix are concerned. He dwells particularly upon the fact that Prof. Ramsay has already assigned certain breccias of Permian age to glacial periods and agency, and that there is good reason for referring the coal-bearing shale of Natal, into which this boulder-bearing clay passes almost imperceptibly, to the Permian system.

For these various reasons, Dr. Sutherland submits that the Boulder-clay formation of Natal should be classed with the Permian glacial breccias of Prof. Ramsay.