of the B uchan D istr id. 1 1 



everywhere the same where I have observed these rocks. 

 They are often slightly porphyritic, with felspar prisms. 

 The following notes on this rock and its varieties, as observed 

 above and below Moore's Crossing, will illustrate its physical 

 and microscopic characters, and its relations to other rock- 

 masses. 



At the eastern end of the section the Snowy River flows 

 over these igneous rocks, which, on the eastern bank, are 

 immediately overlaid by the representatives of the Buchan 

 limestones. I have described in the previous papers already 

 quoted the contact of the Devonian limestones of Buchan, 

 through passage beds, with the uppermost members of a 

 series of felsitic beds, for which I suggested the name of 

 Lower Buchan Beds. Here, however, we find that the marine 

 limestones do not, as elsewhere, rest conformably upon the 

 felsitic beds, but upon massive igneous rocks of a basic 

 character. 



It may be well, however, before speaking further of the 

 mode of occurrence of these rocks at the Snowy River, to 

 consider the results of the microscopic and chemical analyses 

 to which I subjected the collected samples. It will then be 

 possible to make a first step in advance — namely, to deter- 

 mine the class to which these basic igneous rocks belong-. 



I prepared thin slices from samples collected from six 

 different localities. I selected a sample for chemical analysis 

 which appeared to me to be but little altered, and, at the 

 same time, fairly to represent the average character of the 

 rock masses. 



Microscopic Structure of the Rocks. 



The structure of these rocks, as seen in thin slices under 

 the microscope, is generally microporphyritic, and more 

 rarely approaches porphyritic. The ground-mass usually 

 contains, or is partly composed of, some almost colourless or 

 slightly yellowish or brownish basis. I have found this to 

 be in some cases microfelsitic* The basis, where it is at 

 all abundant, contains great numbers of minute dust-like 

 particles. In places these particles coalesce so as to form long 

 and narrow trichites, or often resemble a number of beads 



* Microfelsitic, i.e., consisting of an aggregate of exceedingly minute 

 crystalline particles. See Zirkel. Mikroskopische BesehaffenJieit der 

 Mineralien und Gesteine, p. 280 ; also Bosenbusch PhysiograpMe der 

 Massigen- Gesteine, p. 65. 



