64 Notes on the Geology of the Country around Goulbum. 



The strata evidently nave a thickness of many thousand feet ; 

 but there are great difficulties in the way of determining it even 

 approximately, and sufficient data have not yet been collected to 

 make the attempt. 



CHARACTERS AND OCCURRENCE OE THE OTHER ROCEIS. 



The quartz porphyry represented by the buff colour on the 

 map may next be referred to. It consists of quartz fragments, 

 of irregular form, seldom larger than a pea, set in a compact 

 homogeneous matrix, generally of a dull green colour. 



The following analysis shows the chemical composition of an 

 average sample collected at the Marulan Railway Station. 

 Specific gravity, 275 : — 



Silica 6650 



Alumina ... ... ... ... ... 15"58 



Peroxide of iron ... ... ... ... 513 



Lime 529 



Magnesia ... ... ... ... ... 214 



Potash 2-16 



Soda 2-64 



Loss by ignition ... ... ... ... 1 '31 



10105 

 The dull green colour is due to iron ; as the rock weathers the 

 iron separates and gives it a ferruginous surface like specimen 

 shewn, while the quartz grains fall out and strew the surface of 

 the ground. Its disintegration affords a cold muddy soil. 

 Instances are met with where the matrix is black, and in others 

 white, as the specimens exhibited shew. I think it safest to 

 classify this rock with schists and granites, which geologists now 

 regard as metamorphic rocks. 



According to this opinion the quartz porphyry has originated 

 through chemical changes upon ordinary strata of lime, sand, 

 and mud, perhaps with associated volcanic products ; and its 

 mode of occurrence goes to confirm this. I have seen no instance 

 where the rock could be regarded as having been erupted through 

 the surrounding strata subsequent to their alteration. In all 

 cases it ohades gradually into the strata adjoining it. On 

 Windellama Creek sandstone passes insensibly into it. I have 

 already alluded to the rock which I termed quartziferous schist, 

 as a transition stage between schist and the rock in question. 

 On Groulburn Plains thin layers of it were met with regularly 

 bedded with less altered strata (specimen shewn). But as a 

 rule the quartz porphyry is massive, and never presents any sign 

 of stratification or foliation except at its point of junction with 

 stratified rocks. In the neighbourhood of Marulan the frequent 

 passage of +his rock into granite and syenite proves its alliance 

 with rocks of undoubted metamorphic character. Mr. Clarke 



