572 
SUMMARY OF CURRENT RESEARCHES, ETC. 
scopical peculiarities that enable us to recognize certain 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. 
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 Bocky 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 it 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 granit.’ * 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 pro- 
bably 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 new one, namely mica. This corresponds 
with observations made on similar rocks in other parts of the world. 
The wide distribution of Tertiary leucite rocks in New South Wales is 
a matter of considerable interest. 
The specimens with which this paper deals have been collected over 
widely separated localities. There is material enough in any one 
district for considerable petrological research, but it has been my 
purpose rather to indicate the wealth and variety of our material for 
work, than to give exhaustive details of any one field. Much of the 
matter furnished will, I trust, prove new and of some interest to 
Australian geologists.” 
* Lyell’s ‘ Principles of Geology,’ 11th edition, vol. i. p. 409. 
