230 EEV. J. MILNE CURRAN. 



types of structure as characteristic of particular districts, for 

 instance, a micro-slice of basalt from Orange can always be 

 distinguished from similar rocks at Bathurst. See Plate xxi., 

 figs. 1 and 2. 



In regard to the order of solidification of minerals, as a rule 

 nothing exceptional is to be noted in the material examined. 



The cavities of some basalts are filled, as at Carcoar, with 

 aragonite. There is nothing in the microscopic character of the 

 slices to show that the lime for this mineral was derived from the 

 surrounding rock. On the contrary there are Tertiary volcanic 

 rocks at Rocky Bridge Creek with their cavities filled by opal, 

 and hyalite, and the microscope shows that this material has been 

 derived from the silicates of the rock itself. 



As far as is yet known the great bulk of the eruptive rocks of 

 New South Wales are basic in composition. Intermediate rocks 

 are not common. Acidic rocks are rare. 



There are extensive metalliferous deposits about Cobar, but the 

 microscope reveals no intrusive rocks in connection with these 

 deposits. 



Many of our granites are suffering from a decay called by 

 Dolomieu " la maladie du granite."* Carbonic acid gas, in the 

 air, as has been suggested may have a good deal to do with this 

 process of disintegration, but the microscope shows that to a 

 certain extent the disease is internal. The quartz of some granites 

 is seen to contain numerous gas cavities, and the cloudiness and 

 incipient kaolinization of the felspars is probably due to the 

 absorption of the free gas they once held. 



Many of the rocks called diorites are augitic rather than horn- 

 blendic, and therefore must be classed with diabase. 



In the conversion of a clay-slate to a hornfels, as at Bathurst, 

 the microscope shows that the alteration of the rock consists in a 

 rearrangement of the old minerals, and the introduction of one 



* LyelFs Principles of Geology, 11th Edition, Vol. ]., p. 409. 



