260 



are sometimes developed along certain planes in the slates, 

 and still more commonly in vein quartz. If the vein is nar- 

 row, the rod-like prisms of tourmaline, mixed with quartz, 

 cross the vein at right angles to the rock walls. When the 

 vein reaches a thickness of a few inches the tourmaline be- 

 comes zonal, on either side of the quartz vein, exhibiting 

 parallel dark bands, half an inch to an inch in thickness, 

 which, when closely examined, is seen to consist of very fine 

 bundles of tourmaline needles. Another mode of its occur- 

 rence is in larger prismatic crystals, in quartz, under an 

 arrangement similar to that of graphic granite, the tourma- 

 line taking the place of the felsnar ; whilst the strongly con- 

 trasted colours of the two minerals make a very striking effect 

 when viewed in transverse section. Other accessories are beryls 

 (yellow and blue), which are very common in the Mount 

 Crawford district; garnets, chiastolite, etc. 



XI. — General Considerations. 



Information, at present, is too limited to attempt a full 

 explanation of the great earth movements which built up 

 the Mount Lofty and associated ranges. A few steps in 

 advance, however, have been taken. The base of the Cam- 

 brian series has been determined, and the stratigraphical 

 order of this very thick set of beds (so far as the central and 

 western districts are concerned) is now fairly well understood. 

 The eastern side of the ranges, with its highly metamorphosed 

 rocks, presents greater difficulties, and these await solution. A 

 few facts that will assist in reaching some generalizations may 

 be mentioned. 



It is clear that prior to the movement towards elevation the 

 base of the Cambrians had become depressed to a ^reat depth. 

 This is made evident by the great thickness of the superin- 

 cumbent beds and also by the metamorphosed condition of 

 the beds, which must have sunk to such a depth. It has 

 already been stated that pegmatite veins penetrate the Cam- 

 brian grits in the Barossa district. They are not so nume- 

 rous or on so great a scale as those which intersect the Pre- 

 Cambrian of the same and other districts, but their occur- 

 rence in the lower Cambrian beds is an important point of evi- 

 dence. In the few examples of such intrusive dikes, noticed 

 at Barossa, there was proof of inter-action between the rock 

 mass and the intrusive dike. The latter, along either margin 

 for the thickness of about an inch, showed a modification of 

 crystalline structure as a selvage, approaching the 

 comb-vein structure where the crystallization is 

 developed at right angles to the retaining wall. 

 From the difficulty in distinguishing arkose " clastic 



