218 PROCEEDINGS OP SECTION C. 



This group of rocks is poor in quartz, but still later irruptions 

 are very quartzose. These are found only as dykes, or dyke-like 

 masses in the neighbourhood of the hill. They are composed of 

 quartz and felspar, quartz and muscovite, or muscovite and felspar, 

 and some of them contain schorl in considerable amount. These 

 quartzose dykes and veins are evidently connected with this 

 eruptive area, and are the latest eruptions of the sequence that 

 commenced with the granites. The quartz dykes must be dis- 

 tinguished from the ordinary auriferous or metalliferous quartz 

 veins, and they are of that character which I have elsewhere 

 defined as plutonic quartz.* 



The manner in which these quartz and quartzose dykes are found 

 to occur near, and at the Frenchman's Hill, is worthy of a short 

 notice. In plutonic rocks, such as the granites of that place, the 

 more basic constituents have first crystallized out of the magma 

 in definite forms as minerals, thus leaving it more siliceous after 

 each successive crystallization. I have observed the same in a 

 marked manner, for instance at Noyang, where the first irruptures 

 were of a quartz-mica-diorite, and the later ones of quartz kera- 

 tophyr consisting of merely albite and quartz. 



In such rocks as the granite and quartz diorites, the last con- 

 stituent to crystallize has been the silica, and it seems almost 

 certain that it crystallized from the state of a colloid containing 

 a certain but relatively small per centage of water, some of which 

 may be still found in the minute cavities of such quartz. 



It seems, therefore, that the quartz dykes of the Frenchman's 

 Hill may reasonably be considered as being the residual silica of 

 the plutonic magma, after its basic constituents had crystallized 

 out, and that the siliceous residum was squeezed out into the 

 fissures during the final consolidation of the rocks. On this view 

 no high tempei-ature would be required, but on the contrary, a 

 decreasing temperature must he implied by the conditions 

 postulated. 



The evidence of metamorphism in the rocks of the Fi-enchman's 

 Hill difters in amount, and even to some extent in kind, from that 

 which I have described already in speakingof the crystalline schists. 

 Thus in the granites, felspars are found to be surrounded by a 

 more or less wide margin of detritus. Other felspars have been 

 broken across, and the spaces between the parts filled in by 

 fragments or new materials. Quartz is crushed into minute 

 fragments, or shows strain phenomena when examined by polarized 

 light The mica is distorted, or with its folea spread open. Such 

 effects as these can be observed, but they are local only. This 

 structure of the rocks is that which Rosenbusch has well de- 

 nominated kataklastic. Fissures which have been made in the 

 solid rock have been filled up with minute friction breccias, or even 



♦Notes on the area of Intrusive Rocks at Dargo. Trans. R. Soc. Victoria, xxiii. p. 152. 



