38 Diorites and Granites of Swift's Creek, 



and translucent twisted fibres.* In one slice I found some- 

 what larger plates of this mineral, of a darker green colour, 

 and so situated that I was enabled to make stauroscopic 

 observations, from which I inferred that it was certainly 

 uniaxal. I therefore conclude that this alteration is chlorite, 

 and have thus spoken of it. It is easily decomposed in the 

 slice by warm hydrochloric acid. 



In the production of these masses of chlorite the original 

 form of the hornblende becomes totally lost. These obser- 

 vations lead me to doubt whether any of the masses of 

 chlorite which so frequently are found filling in spaces and 

 lying between various constituents are in any case contem- 

 poraneous constituents of the rocks, but rather in all cases 

 derived from the alteration of amphibole, and more rarely, 

 as I shall point out, of magnesia mica, as well as still more 

 rarely of pyroxene. 



The production of chlorite has in all cases also given rise 

 to the elimination of ores of iron which have been deposited 

 either in the cleavage of the chlorites, or in cleavage or 

 flaws in the neighbouring constituents, or between them. 



A second form of alteration I have observed to arise more 

 frequently in the second variety of amphibole. It much 

 resembles in appearance the talcose pseudo-morphs after 

 hornblende which I have obtained from an amphibolite 

 dyke near Omeo. The extreme forms of alteration in the 

 Swift's Creek rocks have much similarity to serpentine. 

 The first appearances are a lessening of colour, the 

 production of a fibrous structure, and the deposit of iron 

 ores. Further changes result in an aggregate of brightly 

 polarising particles, irregularly or divergently disposed. 

 Figs. 21, 22, are given as illustrative of the above 

 statements. 



Amphibole-anthophyllite. — This mineral is of only local 

 occurrence at the Gum Forest, where it forms almost the 

 whole rock mass. It extends over many acres of ground. 

 It is crystalline massive, of a dark green colour, showing a 

 perfect cleavage in one direction, having a metalloid and 

 somewhat vitreous lustre. Under the microscope, by ordinary 

 transmitted light it is green, or more rarely brown, in colour ; 



* These chlorite fibres differ somewhat in aspect from those other chlorite 

 fibres which so largely make up the mass of the indurated rocks of which I 

 shall speak later on. These latter, where occurring singly, often strongly 

 recall prochlorite in its vermicular shape, as seen in quartz from the less 

 altered contact schists of Beechworth. 



