1870. | SUTHERLAND—-BOULDER-CLAY OF NATAL. 515 
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-mark- 
ings, 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 abun- 
dant 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, with- 
out 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 hill 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 fine- 
ness, while in the other extraneous fragments of rock are brec- 
ciated 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 stratitication. 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, deeplv grooved 
and striated, as if a semiplastic substance, containing hard and 
angular fragments, had been passed over it with considerable 
pressure. ger ee 
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 houlder-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. 
