PETROLOGY. 



By IV. F. SMEETH ALA., A.R.S.M., and J. ALEX. WATT, 



M.A., B.Sc. 



(Plates 1-4). 



Introduction. 



The lithological specimens gathered during tlie Horn Expedition to the 

 McDonnell Ranges include examples of sedimentary, eruptive, and metamorphic 

 rooks. The purpose of this paper is to give merely a short description of the 

 microscopical structure of some of the eruptive and of a few of the most typical of 

 the metamorphic varieties. The region fi'om which almost all the rocks here 

 described were gathered includes portions of the McDonnell and Hart ranges. 

 A general description of some of the granites has already been given in the 

 chapter dealing with the " Geology " of the region ; there will be added liere a few 

 notes only on the felspars. 



Although time has not permitted much detailed work on the specimens here 

 described, there are one or two points to which we should like to draw attention. 

 One is the beautifully developed, though very minute, micro-pegmatitic structure 

 in No. 215. The inter-crystallisation of the felspar and quartz is on so fine a scale 

 that very considerable magnification is needed to recognise it. It was, in fact, a 

 puzzle for some time, until a general similarity to No. 220, where the structure is 

 more coarsely developed, gave us a clue to its i-eal nature. Under moderate mag- 

 nifying powers the appearance is that of a number of compacted grains which do 

 not extinguish under crossed nicols in any position, but are darker in some 

 positions than in others. Owing to this, and also to the fact that some of the 

 grains are built up of sections arranged around some central object, generally a 

 piece of felspar, the different grains are distinguished from one another. 



Another point worthy of notice is the evidence that nmch of the hornblende 

 in the diorites Nos. 25, 32, 19, has been derived from augite. These secondary 

 hornblendes are compact and strongly coloured, the tints ranging from greenisii- 

 blue to brown, while the augite grains are comparatively light in colour. It is 

 questionable whetlier tiiis can be considered a " paramorphic " change, or whetlier 



