The Geology of Coimaidai. 69 



or Silurian. Their origin and maintenance would, therefore, 

 appear to be quite unconnected with the decomposition of the 

 limestone. The irregular and patchy distribution of the lime- 

 stone is, therefore, what might have been expected on this view 

 of their origin, as the springs would in all probability be quite 

 local and even of an intermittent character. The band of ash 

 found in the quarries suggests that the volcanic activity, which 

 was manifesting itself during the deposition of the limestone, 

 may not be unconnected with the development of the CO.^ 

 springs. This point, however, we do not urge, in view of the 

 fact that the springs still continue long after the cessation of the 

 volcanic activity. It is, perhaps, worth while noticing in this 

 context that the course of the Pyrete Creek roughly agrees with 

 the strike of the Silurian rocks in this locality. 



The intercalated bands at the quarry of sand and angular 

 quartz gravel indicate that freshets occurred in the strt ams 

 emptying into the lake in this I'egioii, and the fact that only 

 isolated bones have been discovered suggests that the bones were 

 brought down by streams to their final resting place. , 



The very hard quartzites appear to have been formed by the 

 action of silicious springs. The microscopic examination of the 

 rock strongly coutirms this view ; in the slide i-ounded and sub- 

 angular grains of quartz are set in a matrix of extremely minute 

 specks of quartz, which acts as a cementing material. The 

 peripheral markings of some of the quartz-grains may possibly be 

 explained on the supposition that there has been a growth of the 

 quartz by its taking up some of the silica in solution. There is a 

 good deal of black dust, prol)ably magnetite, scattered through 

 the slide, and it is quite possible that, to some slight extent, the 

 grains are bound together by a ferruginous cement. A few 

 highly refracting and brightly polarising grains visible in the 

 slide indicate, in all probability, the presence of zircons. There 

 is no trace of felspar among the grains. 



Only in this wa.y do we think that the presence of hard, 

 cherty quartzite in the midst of incoherent sand can be explained. 

 The existence of wood opal in the river gravel supports the view 

 that silicious springs occurred in the district. The sediments 

 ultimately filled the Coimaidai basin and spread away to the 

 eastward, where a wide extent of ferrusinous sands and con- 



