CLARKE ON THE IGNEOUS ROCKS OF SYDNEY, N. S. W. 343 



proceeds to describe the different plutonic rocks found in it, and 

 states that they consist of syenite, syenitic granite, protogine and 

 porphyritic rocks of various kinds, and of greenstones, basalt, and 

 trachyte, all, with the exception of the three latter, passing by 

 regular gradations from one to another. The syenites are said to 

 resemble those of Skiddaw, and the syenitic granite that of Guern- 

 sey, while a protogine is described greatly resembling a beautiful 

 rock of the same kind in St. John's Vale, near Keswick. 



At Arthursleigh, the author describes a spot where the face of 

 an exposed cliff exhibits a net-work of quartz veins with dykes of 

 syenitic rock and hornstone ; and not far off a dyke of ironstone, 

 and others of basaltic rocks, amongst which are some in ected 

 trachytes that have been much used for building purposes. 



Having described the position and mineral character of these 

 igneous rocks as they appear en masse, the author then proceeds 

 to allude to some singular instances of intrusive dykes of lime- 

 stone and marble, at a spot known as " Campbells," or " Shepherds," 

 situated on the estate of Arthursleigh just alluded to. These 

 dykes occur in contact with hard large-grained grey syenite, and 

 were seen on the right bank of the river Wollondilly. 



In the first instance mentioned, the width of the dyke is stated 

 to be nearly 47 yards, its dip 50° S.W., and its strike S. 22° E. 

 " Alternations of quartz rock and crystalline white and grey 

 marble compose this dyke ; innumerable lines and scratches mark 

 the edges and face of the marble ; and the quartz has also been 

 subject to a semi-crystalline action, the surface being crumpled or 

 doubled up into parallel anticlinal ridges." There appears to be 

 no line of demarcation traceable between the quartz and marble ; 

 and the two together, after descending into the bed of the river, 

 suddenly curve round and re-enter the granite as a second dyke. 

 Traces of green carbonate of copper are found associated with the 

 other minerals of this dyke. 



The author considers that the scratches and furrows which he 

 has observed, and other phenomena in the line of dip, could not 

 have been in existence before the formation of the present river 

 channel. 



A second dyke is then described in a place where the rocks are 

 thrown into great disorder, and the author details some changes 

 which have produced singular conditions of mineral structure. 

 He also supposes that they exhibit marks of a gradation existing 

 between limestone and quartz. A third dyke of the same cha- 

 racter is then mentioned, in which the constituents of the granite 

 are mixed up with the calcareous rock ; and the author states that 

 near these dykes the granite assumes a distinct character, a 

 greater proportion of felspar and less mica being present. 



At Jaoramin, higher up the river than the spot just alluded to, 

 the structure of the rocks is described as somewhat different, the 

 felspar being less completely mingled with the other minerals, but 

 the rock occasionally passing into porphyry. Where it is not 

 denuded, the rock, however, is here overlaid by a mass of conglo- 



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