MICROSCOPIC STRUCTURE OF SOME AUSTRALIAN ROCKS. 225 



eruptive rocks. I have examined the country about Cobar with 

 considerable care, and have to record that there are no eruptive 

 rocks within a radius of ten miles of the copper lodes and gold 

 deposits of Cobar. When greater depths will have been reached 

 in the working of the " Great Cobar " Mine, eruptive rocks may 

 be met with. But so far as an examination of the country will 

 show, no eruptive rocks can be found. At the Peak five miles 

 from Cobar, there is rock interbedded in the highly inclined slates, 

 which has been considered intrusive. Slide 134 is prepared from 

 this rock. A microscopic examination hardly favours this view, 

 my impression being that the rock is a much metamorphosed 

 felspathic sandstone. 



Slices 54 to 59 inclusive. — Hornfels. The rocks immediately 

 in contact with the granites around Bathurst are good examples 

 of the metamorphic rock, hornfels. In hand specimens it would 

 be hard to believe that the rock is really clastic. It appears as a 

 dense, black, glistening rock, having every appearance of an erup- 

 tive origin. This rock, however changes by insensible gradations 

 in the field, into less altered varieties, until the normal sedimentary 

 rock is reached. Blebs of quartz having a peculiar bluish opaline 

 colour are very common in these rocks. In the slices, by trans- 

 mitted light, it appears as normal quartz. Under the microscope 

 in ordinary light, an abundant development of mica plates and 

 chloritic matter are recognized. The micas are strongly dichroic, 

 very uneven in their outlines, and for the most part, developed 

 in nuidal lines which bend and wave round felspar and quartz, 

 granules. In polarized light, with crossed nicols, the most highly 

 altered of these rocks break up into felspar mosaics, clearly reveal- 

 ing their clastic origin. Slice 58 contains a quartz crystal, around 

 which the micas are developed in the manner referred to. An 

 examination in the field, has satisfied me that these rocks have 

 never been reduced to a plastic condition, but they bear every 

 trace of having been subjected to great pressure, and brought 

 within the zones profoundly influenced by the intrusion of neigh- 

 bouring granites. A study of these rocks of clastic origin teaches 



O— Oetober 7, 1891. 



