790 GENERAL GEOLOGY OF MARULAN AND TALLONG, N.S.W., 



phyry of Marulan. These are nob independent rock-masses, but 

 are all closely related; and these relations exhibit some other 

 remarkable features. As the niap(Plate Ixv.) shows, the Glen- 

 rock grano-diorite forms a roughly elliptical mass about five 

 miles by two miles in extent. Its northern end is traversed by 

 the railway line from near the Barber's Creek viaduct almost to 

 Marulan Station. It yields typical granite-scenery, the surface 

 of the country being dotted with conspicuous granite " tors."^ 

 The deep gorge of Barber's Creek, from 900 to 1200 feet below 

 the level of the plateau, has been cut, for a length of two miles,, 

 out of this very solid rock-material, affording an excellent illus- 

 tration of the enormous power of running water, when the com- 

 parative insignificance of the stream is considered. 



On the east and south the plutonic rock is bounded by older 

 sediments, towards which it shows an intrusive contact. North 

 and east it disappears under the more recent Permo-Carboniferou& 

 conglomerates. The western boundary is the one which calls for 

 comment. In its northern and southern portions the boundary 

 line of the grano-diorite is quite indistinct; the rock passes by 

 insensible gradations through granite-porphyry to quartz-por 

 phyry. There is no doubt at all that the three rock-types 

 represent different phases of the same magma, cooled under 

 different conditions. The transition generally extends over 

 about 300 yards to 400 yards, though here and there it is 

 slightly more abrupt. The cause of this gradation will be 

 discussed below. 



In the central section of the vv^estern boundary, the eruptive 

 rock is in contact with slates and quartzites showing very strong 

 contact-metamorphism. No trace of fossils has been found in 

 these slates, so that their age is quite indeterminate. In the map 

 they have been shaded in the same way as those strata whose 

 age is certainly Silurian; it is, however, quite possible that they 

 are Ordovician, though they lie considerably to the west of the 

 line along which Ordovician rocks are known to occur. This 

 mass of slates extends over an area two miles by one mile, in a 

 continuous mass. All round its borders there occur large and 



