Western Australia. 109 



and frequently, even when they have been discovered, tliere is no 

 residue of the original rock structure. However, since very similar 

 chlontic planes may result both from an acid rock with ferro- 

 magnesians and from a basic one, if neither the character of the 

 felspar can be determined, nor any original structui'e be discovered, 

 discrimination between the two chlorite phases becomes almost, if 

 not quite, impossible, even with recourse to chemical analysis. By 

 careful study and correlation, however, the following classification 

 has been drawn up : — 



A. The quartz-keratophyres, with or without phenocrysts of hornblende, and 



with or without tourmaline. 



B. Those rocks most probably derived from gabbros or dolerites. 



1. Epidiorite group. 



C. The tale-chlorite rocks, derived from amphibolitic or other basic rocks, 



but with indistinct, if any, remains of structure. 



D. Eocks of doubtful origin. 



1. The ' fuchsite ' rocks with or without tourmaline. 



2. Bocks of possible plutonic origin. 



As a rule the identification of rocks belonging to group A has 

 presented no great difficulty, but there are cases in which a deter- 

 mination cannot be made with certainty. Such a case is presented 

 by 1237 2 a. This specimen is a greyish-green rock that is 

 characterized by a considerable amount of scaly chlorite, granular 

 and idiomorphic carbonate, and remains of felspars, which raises 

 doubts as to whether it should be classed as a highly chloritic phase 

 of the porphyrite or as a phase of the dolerite or gabbro. It has 

 been shown recently by Kirk ' how in the neighbourhood of veins — 

 in the particular case copper veins — a rock as acid as a hornblende 

 granite has developed both a chloritic and a sericitic phase. 

 Describing the latter, Kirk remarks: "Grading laterally from the 

 fresh granite is a chloritised facies having many characteristics 

 analogous to those of propylitised rocks. . . ." 



In 12372a, moreover, in the case in which twinned felspar 

 remains admit of measurements of the extinction angles, the species 

 appears to be eitlier albite or albite-oligoclase. The crystals, too, do 

 occur in forms strongly suggestive of their being phenocrysts. The 

 comparatively large amount of chlorite, however, and the large 

 amount of idiomorphic carbonate, with patches of rutile, suggest 

 a relationship witli the more basic dolerites. 



Again, in the area under review, there are several specimens - 

 which consist now of a tine granular mixture of quartz, sericite, 

 calcite, sometimes fine scaly chlorite, and some doubtful particles 

 of leucoxene. These rocks, since no trace of original structure 

 is discernible, have been classed amongst those of doubtful origin 

 (see later on). Now, Kirk, in the paper already referred to, treating 

 of the origin and signifii-ance of sericite states*: "All the above 

 references indicate that where sericite can be certainly identified, 



^ Economic Geology : Mineralization in Copper Veins at Butte, Montana, 

 vol. vii, No. 1. 



- i.e. 12381, S. 610, S. 188. 

 ^ Op. cit., vol. vii. No. 1, p. 57. 



