.258 THE CONSTITLTION Ol- CERTAIN ROCKS OF THE ARCH.EAN AGE. 



•dark-coloured rock with the gneiss and some of its associated 

 rocks above referred to. The comminghng often takes place in a 

 most erratic manner, but on the whole discloses the fact that the 

 dark basic rocks have intruded the other series. Often a clear 

 Jine of contact can be drawn between the two series except where 

 their line of contact or junction happens to be in the vicinity of 

 _ great contortions of the rocks. Throughout local areas, often of 

 great extent, these dark-coloured rocks have been intruded by 

 light grey, yellow or whitish-coloured rocks of a quartzose nature. 

 1 he veins vary in thickness, and in some places traverse or cross 

 and recross the rocks in a most erratic manner, whilst in other 

 places, where these rocks appear foliated, these veins seem to 

 follow the line of foliation in fine parallel bands, with an occa- 

 sional instance of a \ein breaking through the rocks above or 

 below and continuing its course in a higher or lower plane than 

 -that which it orignally had. At places where thev have been 

 swollen out to a thickness of over a foot they have assumed a 

 ^pegmatitic character. I have found that some of them intrude 

 -other more definite formations, such as the Dwyka Conglomerate, 

 and thus their age must be at or subsequent to Fcrmo-Carboni- 

 ferous Times. However, they are a study in themselves, and 

 will be investigated by me at some future time. It will be sufTi- 

 cient here to note their intrusive nature and that they do not 

 appear to be connected with the quartzose veins mentioned in the 

 former part of this paper, but are evidently a younger series. 

 These dark rocks have likewise been intruded bv the type of 

 • diabasic rock referred to above. A pretty scene mav at times be 

 seen in the bed of a river or stream eroded out of this dark rock 

 in those parts where it has been intruded by these parallel veins. 

 The dark rock being more prone to decomposition, has weathered 

 away, leaving the harder type of white intrusive rocks in low 

 parallel ridges. Under the microscope this rock is seen to con- 

 sist of principally triclinic felspar, hornblende, mica and a small 

 quantity of quartz, as well as minor accessories. The mica is not 

 present in anv quantity. .An occasional flake of biotite is all that 

 is to be noted. The hornblende, which is present in some quan- 

 tity, and which with the felspar by far makes up the greater part 

 of the dark rock, is a dark greenish black variety — the same in 

 -e\ery respect to that in the type of basic intrusive rock previously 

 referred to and in the gneiss. The felspars present are whoUv 

 ■triclinic in variety, showing twinning after the Albite Law, with 

 occasional twinning after the Pericline Law. The structure is tyoi- 

 cally holocrvstalline. The felspars are very much decayed. The 

 decomposition products being, as in the other cases referred to 

 above, principally Kaolinite. The felspar is chieflv the variety 

 labradorite, and the other varieties range in the order of basicity 

 vcrv near to this variety. The quartz when it occurs usually has 

 a somewhat circular form and docs not contain on the whole so 

 many Inclusions as in some of the other instances. Among the 

 accessories mav be noticed magnetite. This occurs mostly in 

 granules, though an occasional crystal is to be found. Where 

 -the hornblende has weathered some narts show bleaching with a 

 •separation of magnetite. An occasional scale of haematite is to 



