192 REV. J. MILNE CURB AN. 



A wide field for observation, particularly in microscopic work, 

 is opened up by these suggestions. As far as it-can be made out 

 the order of solidification of the various minerals in the eruptive 

 rocks is studied in the descriptive part of this paper. 



Among the basic rocks included in the slices dealt with are 

 some examples of those interesting forms known as tachylytes or 

 basalt-vitrophyrs. A splendid example of these glassy rocks 

 occurs as a selvage along a joint in basalt near Pratt's Residence 

 Blayney Carcoar Road, New South Wales. In structure, the 

 rock is macroscopically sphserulitic, and of a rich blue-black colour. 

 The sphserulites are of a nut-brown colour and radial in character, 

 and around them are clear absorption spaces. 



In a sphserulitic tachylyte, from Vegetable Creek, New England 

 the sphserulites stand out as warty prominences of a black and 

 dark brown colour on the weathered rock. Films of a decomposed 

 material on the surface of the rock are of a bright blue colour, giv- 

 ing it a very remarkable appearance. Crowds of these sphserulites 

 render the rock opaque in places, even in the thinnest slices. 



In the diorites and diabases too we have to deal with some 

 interesting structures, though perhaps not new to petrologists. 



There is a well-marked diabase rock at Blayney, New South 

 Wales. Microporphyritic crystals of augite are developed in a 

 light-coloured paste of the same mineral with felspar. The absence 

 of well-defined plagioclase will perhaps justify the retention of 

 the term " diabase " for this rock. Some ten miles farther south 

 there is another development of a plagioclase augitic rock, but in 

 this instance, there is an entire absence of a paste between the 

 crystals. It is therefore perfectly holo-crystalline, and, for reasons 

 which are explained further on, I retain the name, gabbro, for 

 this rock. 



The microscopic examination of a number of our dyke-rocks 

 reveals the fact that true hornblendic diorites are not at all so 

 common as is generally supposed. A number of rocks that have 

 passed as diorites are clearly made out to be compounds essentially 



