248 PROCEEDINGS OP SECTION C. 



in Hungary, Transylvania and North-west' America. Seeing 

 breccias and no vesicular lava streams, I supposed that the whole 

 series was an enormous mass of tuffs dipping slightly to the west 

 or north-west, and traversed by dykes of timazite, melaphyre, and 

 dolerite, which looked very different from the surrounding rocks. 

 As much of this supposed tuff was porphyritic, with glassy felspar 

 crystals, I supposed that these portions had been altered by heat. 

 Five or six years later, having had more experience in acidic 

 volcanic distincts, I changed my views and considered these " tuffs" 

 to be submarine lava flows of viscous trachyte.* Professor Gr. H. F. 

 Ulrich, in a letter, informs me that many years ago he determined 

 the hard black rocks occurring in patches among the light-coloured 

 ones in the goldfields, as well as in the range just beyond its limits, 

 as varieties of augite andesite, the former frequently containing 

 hypersthene. 



Last year I made a collection of rocks from this formation, and 

 selected out of it a series of twenty-eight, from which I made 

 sixty-seven thin slices for microscopic examination. 



Sedimentary Rocks. — The lowest beds of the auriferous series 

 at the Thames are seen on the south side of Waiohanga Point, 

 between the point and Waiohanoa Creek. Here the high bluff 

 of the point is formed by the felsite-tuff already mentioned, and 

 has a steep slope on the southern side. On it rests a white felsitic 

 clastic rock with small round pebbles of the felsite-tuff. Then 

 comes a breccia of fragments of blue sandy-slate in a sandstone 

 matrix, which is composed chiefly of f elsite grains, but also contains 

 some quartz and a little chlorite. Upon this is a bed of sandstone, 

 composed of the same materials as the last ; and then follows 

 another slate breccia which passes upwards into the andesite and 

 andesitic breccias, which compose the auriferous series. The 

 exposure is not sufficiently clear to measure the thickness of these 

 basement beds. A much better junction of the two formations is 

 seen on the coast a few miles north of Tapu, between the Mata 

 and Waikowhau Rivers,! but I did not visit this again last year. 



Volcanic Rocks. — None of these are holocrystalline, but all 

 have a trachytoid texture. They show considerable variety, but 

 nearly all are propylites, that is andesites, in which the bisilicates 

 have been altered into hydrated magnesian unisilicates. Different 

 opinions are held among geologists as to whether the name pro- 

 pylite should be retained. This rock bears the same relation to 

 andesite that diabase does to dolerite, and if one is retained, so 

 also should be the other. I am inclined to think that the change 

 indicated by the terms propylite, diabase, melaphyre, and serpentine 

 is worthy of being recorded in the name of the rock, pei^haps by the 



♦Geology of the Thames Goldfiekls. Trans. N.Z. Institute, 1878, Vol. vi. 

 t Second Report on Thames Goldfields, 1868-9, p. 6, and section 3. 



