652 PROF. T. G. BONNET OX SPECIMENS FROM 



rather altered), with a little white-mica, epidote, sphene, iron-oxide, 

 and apatite. The rock appears to have suflFered from, mechanical 

 disturbances, probably at no recent epoch ; but its anterior structure 

 is, I think, commonly well preserved and much resembles that which 

 usually occurs in the old gneisses, to some of which this rock has a 

 macroscopic resemblance. 



6. A pinkish, somewhat porphyritic felstone. Microscopic exami- 

 nation shows it to be a porphyrite, with devitrified ground-mass, 

 the larger felspar-crystals (plagioclase) being rather decomposed. 

 There is nothing noteworthy in the slide. 



7. A rather similar slightly deeper-coloured rock. Also a por- 

 phyrite ; nothing noteworthy. 



8. A darker more mottled felstone. Also a porphyrite, but 

 differing from the others in containing a fair amount of a greenish 

 mineral. This, under the microscope, is seen to occur in irregular 

 patches, sometimes adumbrating the form of a pyroxenic or micaceous 

 mineral. That which dominates in these patches is generally 

 a strongly dichroic, olive-green mineral in small folia, probably a 

 chlorite, associated with viridite and sometimes calcite. Probably 

 all these are of secondary origin, indicating the former presence of 

 a ferro- magnesian silicate. 



9. A rock bearing some resemblance to the last named, but 

 with much pyrite in the hand-specimen. This is less abundant in 

 the slide, which shows the rock to be also a porphyrite, bearing 

 some general resemblance to the last described, except that the 

 green patches are yet more indefinite in outline and more in- 

 determinate in structure. So far as I can ascertain, they are largely 

 composed of granules of an impure epidote and belonites of a 

 greenish mineral, probably hornblende. 



10. A moderately fine-grained, pinkish, granitoid rock ; the 

 microscope shows it to be composed of quartz, felspar (microcline, 

 orthoclase and oligoclase ?), hornblende, biotite, and apatite, with 

 probably one or two small zircons. There are some slight indications 

 of mechanical disturbance, but none of marked crushing, and the 

 structure of the rock recalls that of the Archaean gneisses rather than 

 of the normal granites. 



So far as I can form an opinion from the slides and specimens 

 alone, I should say that Nos. 1, 3, 5, 10, with possibly 2 and 4, belong 

 to an ancient series of rocks, which, even if wholly or in part of 

 igneous origin, assumed their present mineral structure and condition 

 at an epoch remote from the present. Some of them resemble rocks 

 known to be Archaean, and they do not resemble (so far as my ex- 

 perience goes) any which are indubitably Post-Archaean : 6, 7, 

 8, 9 are certainly igneous and of more recent date, though I should 

 doubt if they were not at least of early Tertiary age. 



EXPLANATION OF PLATE XV. 



Fig. 1. Section of flattened garnet, showing mica-schist on right-hand side. 

 X40. 

 2. Section of flattened garnet, X 40. 



