60 Diorites and Granites of Swift's Creek, 



bolite, in which the hornblende is often replaced by magnesia 

 mica, when the quartz diorite in this their extreme form 

 becomes, so far as physical character is concerned, true 

 plagioclase granites. In leaving the Gum Forest and 

 ascending toward the Tambo River, either by the Long 

 Gully or the Sheep Station Creek,* a large tract is found to 

 be entirely occupied by granitic rocks. We have here 

 the usual physical character of country seen in granite 

 areas — namely, rounded hills, but occasionally varied by 

 summits crowned by rocks, or slopes dotted by weathered 

 masses and tors of granite. The rock itself is usually a 

 granitite or an amphibole granite, with varieties connecting 

 both. Such granite rocks as these are found extending to 

 the eastern contacts, where they are seen to have been in- 

 truded into the regional metamorphic schists and the less 

 metamorphosed Silurian sediments of the district. The 

 phenomena of contact in the granites are everywhere such 

 as these I have described — namely, a strongly marked gneissic 

 structure of the intrusive rocks near the contact, and per- 

 haps an increase of potash felspar in them, due, I suspect, to 

 the admixture of portions of the sediments which have been 

 absorbed. 



It soon becomes evident, in tracing out the contacts of the 

 two groups of rocks at Swift's Creek, that the igneous 

 masses are intrusive into and, therefore, younger than the 

 sediments. The actual contact is, however, not always easy 

 to point out, for the gneissose margin of the former and the 

 greatly metamorphosed beds of the latter often interlock. 

 But I have found, upon repeated examination, that those 

 beds, such as aplite and hornfels, which alternate with the 

 intrusive rocks, are, in fact, those extreme edges of the sedi- 

 ments which have remained attached to and in the consoli- 

 dated igneous mass, while their extensions towards the 

 sediments to which they belong have been denuded. I can also 

 perceive that more than one irruption of igneous rocks has 

 taken place, for I have observed that dykes nearly approach- 

 ing the character of the amphibolites penetrate the granite 

 and gneissic diorites. Similarly, I have observed that com- 

 pact diorite dykes penetrate them, while the amphibolites, 

 the amphibole-gabbros, and the anorthite diorites, so far 



* The Sheep Station Creek falls into the Tambo River. The Sheep Station 

 Gully is in the Gum Forest, and is the western branch of Riley's Creek. It- 

 is not lettered on the map. 



