﻿Vol. 64.] CAMBRIAN AGE IN SOUTH ATJSTEALIA. 239 



lY. Description of the Beds. 

 (a) General Features. 



The beds which give evidence of glacial origin may be described 

 as consisting mainly of a groundmass of unstratified, indurated 

 mudstone, more or less gritty, and carrying angular, subangular, 

 and rounded boulders (up to 11 feet in diameter), which are 

 distributed confusedly through the mass. It is, in every respect, a 

 characteristic till. The included stones sometimes occur in pockets 

 or groups, but the rock never becomes a typical conglomerate. 

 Coarse angular grits and quartzites often occur in the form of 

 irregular deposits, mixed with the finer groundmass, and these may or 

 may not carry boulders. In places the beds become highly siliceous 

 and very close in the grain, probably owing to the introduction 

 of silica-charged waters, which have given rise to much quartz- 

 veining at some points. In a similar manner, in the neighbourhood 

 of impure calcareous zones, calcite or quartz-calcite veins may occur. 

 Where the till is in its normal condition, it generally exhibits 

 numerous small cavities, from which the more perishable included 

 rock- fragments have disappeared, leaving behind them external 

 casts in the matrix. 



In most sections there are more or less regularly-stratified beds 

 or bands, which occur at various horizons in the till. These may 

 be of quartzite, finely-laminated slate, or limestone. The last- 

 named seldom exceed 2 or 3 feet in thickness, are often gritty, 

 and contain angular stones. The slates, in their fine lamina- 

 tion, bear some resemblance to the overlying Tapley's-Hill Beds ; 

 but, while the latter usually maintain an exact parallelism of 

 deposit, even to the minutest lines, the slates of the till exhibit 

 transverse bedding to a remarkable extent. It is only by the 

 presence of these regularly-bedded intercalated deposits, that the dip 

 of the thick unstratified till can be judged. 



The thickness of the glacial series has been proved up to 1500 

 feet. The interbedded members may divide the till proper into 

 two or more divisions, yet in all cases the latter shows a prepon- 

 derating thickness in the sections. The more remote northern 

 outcrops have not been studied so fully as those in the south ; but 

 observations, so far as they have gone, seem to indicate that in 

 the far north the boulder-clay has been interrupted by regularly- 

 bedded deposits to a greater extent than is characteristic of the 

 beds in the southern parts of the country. 



When the till is of an earthy, or non-siliceous, nature, it frets 

 away rapidly by weathering, and sometimes forms cavernous 

 shelters in the faces of cliffs. Under such circumstances, with 

 a friable and freshly-exfoliated surface, its resemblance to the 

 Pleistocene Boulder-Clay of Europe is very striking. 



Q. J. G. S. No. 254. R 



