Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/713

 that of a clay which has been deposited by aqueous action, and afterwards metamorphosed by heat, pressure, and chemical action. In places the deposit exhibits the most unmistakable ripple-markings, which seem to indicate that the wind, or other force, which agitated the water during the period of deposit, acted at different times in different directions. The thickness of the deposit varies in different situations, but in some places amounts to as much as 1200 feet.

The clay is compact and tenacious, and unfavourable to abundant supply of water. Borings into its substance remain dry to almost any depth. The surface-soil, which is associated with the formation, is stiff and non-fertile until it has been loosened to a considerable depth and thoroughly worked.

The boulder-bearing clay rests generally upon old Silurian sand- stones, which in their turn are based upon granite and gneiss, and which in that connexion constitute the peculiar South-African table mountains. Upwards it passes first into newer shales, and through them into the sandstones and shales, which are associated in Natal with Carboniferous deposits. The transition is very gradual, without any distinct line of demarcation, and often stretches through a quarter of a mile of debatable ground. The transition is very well seen in the Town lull directly above Maritzburg. The only real difference between the shale and Boulder-clay seems to be, that in one of them the matrix is of uniform homogeneity and fineness, while in the other extraneous fragments of rock are brecciated in the argillaceous substance. Near the Umpambinyone and Umzinto rivers the boulder-bearing clay passes into beds which very nearly simulate the condition of true slate, but have their lines of cleavage in the direction of, instead of transverse to, the general stratification. In one instance, in this situation, there is a fine flagstone very closely resembling the Caithness Sandstone. Ripple-markings are plentifully developed where the Boulder-clay passes into these fine slates and flagstones.

The old sandstones which lie immediately beneath the Boulder- clay have their upper surface, in many instances, deeply grooved and striated, as if a semiplastic substance, containing hard and angular fragments, had been passed over it with considerable pressure.

The rocky fragments imbedded in the Boulder- clay formation are all of them of the character of the rocks that are contemporaneous with, or inferior to, the Silurian Sandstone. Fragments of a higher series are never found in the matrix. In some instances very ponderous rock-fragments are found as much as fifty or sixty miles away from the sources of their derivation.

Mr. Bain, in his communication to the ' Journal of the Geological Society ' in the year 1856, has very exactly expressed the condition and relations in which this boulder-formation is found in Natal, flanking the sandstone-beds which cap the granite and gneiss. It is, however, certainly very questionable whether this so-called Claystone Porphyry of Mr. Bain is really of the nature of porphyry.