BY W. N. BENSON. 671 



So far no examples of bending, as an original structure, 

 (" Schlieren ") have been obtained. 



No rocks yet found in the belt have escaped chemical altera- 

 tion. The processes of alteration may be classed as : —(a) Normal 

 serpentinisation ; (/?) Change to antigorite ; (y) Carbonation ; 

 (8) Silicification with oxidation. 



The two latter occur together, the one or the other predominat- 

 ing, and both are subsequent to the two former. The products 

 of these processes further differ according to the degree of 

 pressure they have undergone during their change. 



(a). The formation of normal serpentine has been so often de- 

 scribed, that a brief note will here suffice. The process affects, 

 but in different ways, olivine, enstatite, and diallage. Olivine 

 produces its well-known mesh-structures, with, frequently, sepa- 

 ration of magnetite, which is generally deposited in the strings 

 of the mesh, along the first-formed cracks; occasionally these 

 cracks are quite free from magnetite, and the deposition is in the 

 interstices of the mesh [e.g., N.T., 132]. Enstatite forms large, 

 clear, platy pseudomorphs, with occasional deposition of magnetite 

 in the cleavage-cracks. Usually the enstatite becomes very 

 cloudy while the change is in process, but the finished product, 

 bastite-serpentine, is quite clear. The alteration of the diallage 

 is not exhibited by any of the New South Wales serpentines, 

 further than the grain becoming cloudy, and a small amount of 

 serpentine forming in the narrow cracks. The chromite is quite 

 unaltered during this change, and in all subsequent changes also. 



There are two types of massive serpentine, which are more 

 clearly distinguished in the field than under the microscope. 

 One marks a strong outcrop, with a rough weathering surface 

 stained red or brown. The oli vine-serpentine is etched out on 

 weathering, the bastite and talc remain in high relief. On 

 fracture, the rock is dark, often poikilitic, and frequently con- 

 tains small, irregular, white patches of steatite, which is exceed- 

 ingly finely divided and nearly isotropic, owing to mutual com- 

 pensation. The chemical and physical difference between this 

 type of rock and its derived soils is shown by the difference in 

 soil-colour, here reddish, and in the vegetation. 



