ON THE ROCKS OF THE HAURAKI GOLD-FIELDS. 261 



is difficult to explain, but it is no doubt connected with the absence 

 of f ragmental volcanic rocks, and both point to a viscid anliydrous 

 condition of the lavas on tirst extrusion. 



Surface decomposition has penetrated downwards in a very 

 irregular manner and has left, in places, isolated masses of 

 dark undecomposed rock surrounded on all sides by the paler 

 products of decomposed. A very good example was seen in a 

 shaft in the Caledonian Mine which was sunk vertically for forty 

 feet alongside a hard mass of decomposed rock.* Also at the 

 three hundred and fifty feet level of the same mine a hard patch 

 was met with which was supposed to be isolated. Mr. Cox gives 

 another explanation of the position of this last mass,! but he 

 seems to liave overlooked Mr. Davis' section. One of the early 

 drives in the Moanataiari Mine passed under a large isolated 

 block of hard rock,| and in the Puriri District many large 

 spherical masses of undecomposed andesites occur in the de- 

 composed portions. § 



This irregular decomposition may account for some of the hard 

 masses which still appear at the surface, but neverthle-^s I think 

 that many of them are dykes. This was my opinion in 1868, but, 

 with the exception of Mr. Davis, 'the Government Geologists who 

 have reported on the Thames since 1870, have treated the whole 

 series as a volcanic formation of great thickness without any 

 contempoi'aneous dykes, but possibly with some belonging to a 

 later and quite dift'erent period. Indeed Mr. Cox seems to think 

 that the hard portions are regularly bedded with the softer 

 portions and dip to the W.N.W. at an angle of about 26"" with all 

 the regularity of a sedimentary formation, and as quite unbroken 

 by dykes. The microscopic examination of these rocks has, 

 however, tended to confirm the idea that some of the hard massc^s 

 are true dykes. I base this opinion on the absence of magnetite 

 dust in the base and its collection into large grains ; on the 

 grouping of the pyroxene crystals ; and on ophictic structure ; 

 for all these things prove that the cooling process was slow and 

 that during the whole time the mass of rock was at rest. I will 

 bi'iefly describe a few of these dyke-like masses. 



1. — On the shore a little north of Tapu Creek a dyke, ten feet 

 tliick and running north-west, is plainly to be seen. This dyke is 

 a hornblende andesite with an abundant pale brown ground-mass 

 containing rather large grains of titaniferous magnetite. The 

 hornblendes are partly fresh and partly changed to chlorite. 

 S.G. = 2.680. 



2. — A little way up the Moanataiari Creek, on the south side, 

 and below the old Point Russell Claim, there is a mass of hard dark 



•Davis, Reports Geological Explorations, 1870-1871, p. 63, and section 6. 

 t Reports Geological Explorations, 1882, p. 30, and fig. 

 t Reports Geological Explorations, 1868-9, p. 31, fi^. 1. 

 § Reports Geolo^'ical Explorations, 186S-9, p. 35. 



