and their Contact Zones. 59 



(3.) The Interrelations of the "Rock Masses. 



The Igneous Rocks. 



In passing across the more or less well defined contact 

 boundary between the sedimentary and igneous rocks of 

 Swift's Creek to the area of the latter, I have usually 

 remarked these to consist of rather coarse crystalline granu- 

 lar compounds of quartz, felspar, mica, and hornblende, in 

 which a gneissic structure is always more or less marked. 

 These peculiarities of structure extend for various distances, 

 and their extent depends probably not only upon local 

 differences of crystallisation and cooling, which may well be 

 supposed not to have been everywhere the same, but also 

 evidently in many places on the irregularity of the contact 

 plane, in respect to which the present surface rocks would 

 have been situated at greater or less distances. Those rocks 

 first met with are varieties of plagioclase gneiss, or more 

 properly gneissose quartz diorite. In proceeding still 

 farther beyond the contact towards the central masses, an 

 increasing change is found. The granitic rocks become 

 darker in colour by the increased amount of magnesia mica ; 

 and, at the same time, they are harder and tougher. The 

 gneissic structure passes into an irregular crystalline granu- 

 lar structure. The texture varies much from a fine-grained 

 rock, composed mainly of minute scales of black mica and 

 quartz, to a coarse compound of quartz, felspar and a little 

 magnesia mica, or blackish green to black hornblende. 

 Other varieties intermediate between these occur, either 

 singly or else even irregularly massed in the rocks them- 

 selves. In proceeding southward the rocks become more 

 dioritic in character, and parts are met with along the 

 southern boundary of the intrusive masses where hornblende 

 so preponderates as to produce rocks such as the amphibo- 

 lites. These rocks extend along the contact from Eureka 

 to Eiley's Creek, and it is here that the peculiar am- 

 phibole-anthophyllite rocks are met with. Still further 

 round the contact, and at the Sheep Station Creek Gap, are 

 the anorthite diorites. The contact phenomena of these 

 rocks and the sediments do not differ from those found 

 where the intrusive rock is of a more granitic character. 



Thus, the area of the Gum Forest and of the upper part 

 of Swift's Creek is occupied almost exclusively by intrusive 

 rocks of the classes quartz diorite, diorite, and amphi- 



f2 



