274 Transactions. — Geology. 



resting on palaeozoic rocks, and cut by numerous dykes of dioiite,* but 

 occasionally of trachyte. This tufa appears as a soffcish, grey, coarse-grained 

 rock, weathering white, and sometimes much stained with peroxide of iron. 

 Where cut by dykes it is hardened for considerable distances, and much 

 altered in appearance. As might be expected, however, from its origin, it 

 varies a good deal in character, often containing rounded blocks of diorite one 

 or two feet in diameter ; indeed, in three or four places it passes into a true 

 conglomerate, while occasionally small angular stones are found in it forming 

 a breccia ; these latter, however, are very local. The whole of the rock, 

 including some of the dykes, is much impregnated with iron pyrites," 

 (Geological Report on the Thames Gold Field, 1867, t) I there state that 

 auriferous quartz veins had been found in eight places in the trachyte tufa, and 

 that I considered that " this tufa is probably of tertiary age, and not older 

 than the "Waitemata series." In this report, therefore, I agree with Dr. Hector 

 that the gold reefs are situated here, as at Coromandel, in " grey pyritiferous 

 rock," but consider that at Shortland this rock is only a part of the vast 

 overlying accumulations of tertiaiy trachyte tufa. 



In November and December, 1868, I again visited the Shortland and 

 Tapu districts, and in the report I furnished (Second Report on the Thames 

 Gold Fields. Geological Reports, 18.68-69, p. 15) I reiterated my former 

 views, adding that the lower part of the tufa formation had been metamorphosed 

 <'into a hard, green, pink, or purple felspar-porphyry, or more rarely into a 

 hornblende-porphyry." I also reported that in the Tapu district some of the 

 lodes were in the older slate formation, which is there nearly vertical, but that 

 most of them were in a trachyte tufa and breccia. 



In December, 1869, I visited Coromandel for the first time, and reported, 

 in Januaiy, 1870 (Geological Reports, 1870-71, p. 2), that by far the larger 

 part of the country, including the central dividing range, was composed of 

 tertiary trachyte rocks, like those at Shortland, lying unconformably on a 

 basement of older slates, and that all the gold mines were situated in the 

 trachyte tufa. Thus considering the trachyte dykes of Di'. Hector as part of 

 the tufa, and as having nothing to do with the palaeozoic slates. 



Meanwhile, between 1868 and 1870, Dr. Hector had also several times 



* This is a mistake, the dykes are of dolerite, melaphyre, and timazite. 

 •j* The map accompanying this report was altered without my knowledge, and it now 

 agrees neither with the report nor with my opinion. 



[Only the reference to the map was altered without consiilting Captain Hutton, as it 

 appeared to be inconsistent with the report as altered by him before it was sent to press. 

 It originally stood thus : — 



Alluvium. 

 Tertiary j Jrachytic tufa. 



*' ( banclstone. 

 Palseozoic ? Trachytic tufa, — J. Hectoe. ] 



