632 DR. J. A. THOMSON ON THE [Dec. I913, 



where the rocks are contact-altered, and is generally disposed in 

 small tufts with very irregular boundaries to the individual 

 crystals. The spaces between the tufts are filled with very fine 

 aggregates of zoisite, sphene, and quartz, and these minerals also 

 occur in nests and venules along with chlorite and carbonates. 

 Felspar occurs generally in platy crystals giving long lath-like 

 sections, in which no extinction-angles greater than 10° have been 

 observed for symmetrically-disposed albite-twins. In many of the 

 rocks felspars appear to be absent, but have possibly escaped 

 detection in the fine aggregates of quartz and zoisite. 



Fine-grained amphibolites showing contact-metamorphism are 

 found in some dumps near Monument Hill, 3 miles south of 

 Boulder Post-Office ; and, as these dumps are surrounded on 

 all sides by dumps containing porphyrite, it is probably to the 

 agency of the latter rock that the contact-alteration must be 

 ascribed. The hornblende is no longer pale, but is vivid green 

 with bluish tones, similar to those displayed, by the actinolite- veins 

 which ramify through the Cornish spilites in the contact-aureole 

 of the Land's-End granite. Both the hornblende and the clino- 

 zoisite are reconstructed into larger and more euhedral crystals, 

 and the sphene occurs no longer in granules but in wedge-shaped 

 prisms. Biotite has developed to some extent within the horn- 

 blende, while the clinozoisite contains kernels of orthite. Finally, 

 the rocks are penetrated by venules of quartz, clinozoisite, and 

 actinolite. All the above-described features are characteristic of 

 contact-alteration in amphibolites. An additional feature in some 

 of the specimens is the presence of small ellipsoidal white patches, 

 measuring 24 cms. in their greatest diameter, which give a con- 

 cretionary aspect to the rock. These consist of hornblende and 

 zoisite, like the rest of the rock ; but the hornblende is in less 

 amount and in stouter prisms, while zoisite preponderates as a 

 dense mosaic. 



Fine-grained amphibolites, in many respects similar to those of 

 Kalgoorlie, have a fairly-wide distribution in Western Australia. 

 Among the rocks which I described in 1909 from the "West 

 Pilbara Goldfield, 1 there are three that may be classed here 

 (G.S.M. 6405 & 6415, Weerianna; 6429, 2 miles north-east of 

 Mount Marie, near Koebourne). Because of the lack of relict- 

 structures and the greater metamorphism which they showed in 

 comparison with the coarser amphibolites, it was then suggested 

 that they formed an older series, comparable with the Lewisian of 

 Scotland. A re-examination of these rocks in the light of the 

 experienced gained at Kalgoorlie shows that the greater structural 

 metamorphism is due to contact-alteration, and that they were 

 probably before then fine-grained amphibolites similar to those of 

 Kalgoorlie. A better analogy of their probable relation to the 

 coarse-grained amphibolites is illustrated by that of the Old Lizard 

 Head Series to the Lizard serpentines and gabbros. 



In the Cue and Day-Dawn Goldfields, Mr. H. P. Woodward & 



1 Bull. Geol. Surv. W. Austral. No. 33 (1909) pp. 132, 137, 141, & 143; 

 also fig. 50, p. 141. 



