690 GREAT SERPENTINE BELT OF NEW SOUTH WALES, Hi., 



colourless portion. This is due to the fact, that the whole of the 

 felspar has been replaced by prehnite, which forms very irregu- 

 larly shaped, interlocking patches. The crushing of the rock 

 has induced a very undulose, sometimes anomalous, extinction in 

 the prehnite, making its determination difficult. In unstrained 

 areas, and especially on the borders of the slide, where the 

 cleavage shows better, the following observations were made, 

 which determine the mineral to be prehnite. The refractive 

 index is greater than anorthite, but less than pyroxene, the ex- 

 tinction being parallel to the cleavage. The optical character is 

 positive, the optic, axial angle slightly greater than 125°, and the 

 axial plane is perpendicular to the cleavage. The double refrac- 

 tion-colours are often low, but tints, higher than those of the 

 highest colour for the diallage, have been observed. Sometimes 

 rocks such as this are porphyritic, with prehnite as phenocrysts, 

 e.g., M.B., 236, a dyke on Crow Mountain. 



Another curious rock forms a dyke in the serpentine on the 

 road near Wood's Reef [M.B., 83]. It forms a dense, red, 

 weathering skin on an interior of hard, aphanitic, buff-coloured 

 rock. This consists of small twinned pyroxenes, sometimes with 

 the herring-bone structure, lying in a ground-mass of finely 

 granular, almost homogeneous garnet. A similar but more coarse- 

 grained rock, [M.B., 28] differs in being pale green in colour; the 

 garnet is clouded with dusty, oblong areas, both large and small, 

 the appearance of which strongly suggests that they represent 

 the original felspar-laths and phenocrysts. The rock was pro- 

 bably somewhat sheared before its alteration. 



Finally may be mentioned, a dyke-rock [M.B., 185] occurring 

 at Paling Yard. It is greenish, recalling a dunite, but consists 

 of kaolinised felspar with secondary albite, epidote, and streaks 

 of chlorite. 



The mineralogical changes in these rocks are closely analogous 

 to those that have been undergone by the gabbros, and are clearly 

 pressure-effects, which, as will be seen, have not been suffered by 

 any subsequent rocks to anything like such an extent, It must, 

 therefore, be taken that these dolerites were closely associated 



