﻿Yol. 64.] GLACIAL BEDS OP CAMBRIAlf AGE IN SOUTH AUSTRALIA. 247 



(d) Effects of Mechanical Strain. 



In some localities, such as the Sturt Valley and the Petersburg 

 Eanges, for example, the beds have been in the zone of great fold- 

 movements and give evidences of strain. A rough kind of cleavage, 

 at a high angle, has been developed, causing the rock to split up into 

 coarse flaggy masses, which weather into serrated edges and often 

 form bold cliff-faces. The coarseness and unevenness of the 

 ground-mass prevent fine lamellar division on the cleavage, but 

 the rock usually exfoliates in smaller or larger flakes along the 

 planes of cleavage. The groundmass has by pressure become 

 kneaded round the included stones ; and, in the process of flaking 

 off, these stones form a bulging centre to lenticular flakes, regulated 

 in size according to the size of the included boulder. 



The pressure that has induced cleavage in the bed has also shown 

 its effects on the boulders. These, especially the boulders of elon- 

 gated shape, have undergone a partial revolution in the bed, so as 

 to take a position in which the longer axis has become parallel 

 with the cleavage-planes. In this movement some of the stones 

 have become slightly distorted, and many show the effect of friction 

 in the form of pseudo-striation on exposed surfaces. In the case 

 of angular fragments the edges are sometimes bevelled off, and 

 marked by strong parallel striae covering the entire face of abrasion. 

 In other instances, these lines of scour diverge radially from a pro- 

 minent part of the boulder, as though the latter had become scraped 

 in consequence of a differential movement in the groundmass as 

 compared with the boulder, which latter would offer greater resist- 

 ance to movement. This pseudo-striation, owing to movement 

 in the bed, differs from glacial striation in its uniform character 

 and extent. The lines are of equal size and depth, are parallel 

 one to the other over wide surfaces, and are often flanked by ridges 

 (see fig. 6, p. 246) ; while the glacial striae are generally individual 

 or patchy in their occurrence, of varying intensity, and divergent in 

 direction. By these distinctive features the two classes of striation 

 can be easily differentiated. 



Another mass-movement has also taken place (probably subse- 

 quent to that which induced the cleavage), in which mechanical 

 strain operated along the planes of cleavage. This movement has 

 exerted a drag on the boulders, and caused a great number of them 

 to fracture across their short diameters and at right angles to the 

 cleavage. These fractures are often very numerous, dividing the 

 stone into laminae, with gaping interspaces, which are sometimes 

 filled with secondary minerals, such as quartz or calcite. Many 

 are, however, free from deposit which might occur from the 

 removal of soluble minerals that at one time occupied the cavities. 

 Examples are not uncommon in which deposits of fibrous calcite 

 have been partly removed in this way. Similar cross-fractures 

 are seen in many of the fragments which make up the coarse grits 

 of the glacial beds, especially when the stone is viewed on a joint- 

 face. 



