MICROSCOPIC STRUCTURE OF SOME AUSTRALIAN ROCKS. 181 



The more modern method is to place a thin slice under the 

 microscope and outline the field with a camera-lucida, and then 

 weigh the various parts of the dissected drawing upon a chemical 

 balance. 



In my own work I have adopted the method originally suggested 

 by Dr. Sorby.* I take one or more characteristic micro-photo- 

 graphs and cut out the portions representing the different con- 

 stituents and weigh them separately. It is often found difficult 

 to cut out the parts exactly, so that a liberal margin must be 

 allowed for error. I notice too, that in some of our basalts, the 

 granular augite in the base is of a rich brownish-yellow colour. 

 This is so non-actinic that with ordinary exposures the negative 

 is hardly affected. The result is that in a finished print it is 

 nearly impossible to separate the grains of augite and magnetite, f 

 Prolonging the exposure is one way of overcoming the difficulty, 

 but then the negative is so over exposed for felspars and olivines 

 that it is useless except for the one purpose of separating the two 

 minerals just named. In these cases the use of a polarizing prism 

 or of both nicols inclined will be found a great help when con- 

 trast is desirable in the micro-photographs. 



The micro-photographs which illustrate this paper were made 

 from the rock-slices that are submitted with it. Tn some instances 

 where particular parts only of the microscopic field were required, 

 drawings have been made from the micro-photographs. 



In detecting glassy matter in rocks I have found the quartz 

 plate of much assistance. Technically, the isotropic character of 

 the glass should suffice for its detection, but in practice it is found 

 much easier to catch the interference colour imparted by the 

 quartz than to recognise a singly refracting patch. 



Some idea of the nature of the glassy portions of rocks may be 

 gained by prolonged treatment with hydrochloric acid. I have 



* Penological Notes on some North of England Dykes, J. J. H. Teall, 

 Quart. Journ. Geol. Soc, Vol. xl., p. 216. 



t It was the late Richard Daintree, c.m.g., who suggested the use of 

 micro-photographs in this connection — Geol. Mag., Dec. III., Vol. v., p. 3. 



