PEOCEEDINGS OF SECTION C. 373 



Hochstetter mentions many occurrences of breccia as similar to those 

 of Waitakerei and Whangarei Heads, and the probability is that they 

 contain the same rock — h\^ersthene augite andesite : but I have had no 

 opportunity of examining them, and no work has been published on 

 them at present. At Wliangaroa this rock occurs in the breccia, with 

 hypersthene hornblende andesite. 



There is in the Coromandel Peninsula another series of volcanic 

 rocks widely developed that has generally been regarded as much 

 younger. It consists of rhyolitic rocks of very variable structure and 

 appearance. Quartz is relatively rare in these rocks, and the felspar 

 is often of a triclinic variety. In many instances the excess of silica 

 is in the form of tridymite. The structiire is very variable. At Paku 

 Island, in particular, but in many other localities as well, spherulites 

 of a very large size — not infrequently lin. in diameter — constitute the 

 main mass of the rock. The spherulites have the pseudopodial form 

 of Rosenbusch. They have been described by Rutley and by Sollas, 

 who speaks of them as cerviform. Rutley thinks they are due to 

 devitrification. Sollas says they are the result of decomposition. 

 They have been well figured by Rutley, and McKay has made good 

 photographs of them, which have been published with the notes of 

 Professor Sollas on the rocks. To me, the spherulites appear to be the 

 original structures, though I cannot at present offer any explanation 

 of their formation. 



Amongst the larger spherulites, and between their arms, are 

 numberless microscopic ones. In many rocks these occur without the 

 large ones, and may be arranged in rows or layers, when they give a 

 very prominent banded appearance to the whole rock, and under the 

 microscope they merge into axiolites. In addition, there is a third 

 type of spherulite of intermediate size and opaque, except in very thin 

 sections. They are always perfectly spherical, and the size usually is 

 that of a pea. The causes that have resulted in these different struc- 

 tures appear to me to be beyond discovery by microscopic observations. 

 Very careful chemical work seems more likely to give a clue to the origin 

 than any study with the microscope. So far as I have seen, the rhyolites 

 with coarse spherulites have little glassy residue. This appears to be 

 the case also in the rocks composed of the spherulites of microscopic 

 dimensions. In the third type of spheruUtic rhyolite there is usually a 

 large amount of glassy base, and the rock becomes a spherulitic pitch- 

 stone, and varies into a true obsidian, like that at Mayor Island. The 

 glass is usually filled with trichites, which are clearly curved rows of 

 globulites. 



The rocks with pseudopodial spherulites are usually destitute of 

 crystals of any mineral except quartz, in small nests of irregular grains. 

 The others usually have crystals of trichinic felspar, hypersthene, and, 

 less frequently, hornblende or biotite. 



From the Coromandel Peninsula rhyolites extend over a very large 

 area to the south and west, but the spherulitic character is less developed 

 than in the Coromandel, though examples are not infrequently found. 



