238 PROCEEDINGS OF SECTION C. 



tuff, the whole having been derived from the grinding up of the 

 underlying quartz-porphyry during volcanic eruptions." Specimen! 

 No. 1 (a) is also a tuff formed of a moderately hard base of a dull 

 reddish-brown colour, resembling a half-burnt brick, in which are 

 embedded whitish-grey and brownish-grey fragments, varying in 

 size from that of a pin's head to that of marbles. These shew 

 every gradation of shape from the round to the angular. They 

 consist partly of crystalline rocks such as basalt and felsite, and 

 partly of claystone. The fragments of dark grey basalt shew 

 glistening crystals of limonite pseudomorphous after magnetite, 

 and most of them consist of a thick shell enclosing a small kernel 

 which is readily detachable from the shell. Felted crystals of 

 kaolinized felspar of a pale greenish-grey or rusty-bi'own colovir 

 can be clearly seen under the microscope The spaces between the 

 felspars are partly filled with black crystals of limonite. The 

 hollow in one of these basalt lapilli seems of primary origin, so the 

 fragment is probably a hollow bomb. I cracked open the kernel 

 in it and found that it was composed of a reddish-grey amygdule, 

 quite unlike the decomposed basalt forming the shell around it. 

 Most of the large grey fragments, from one-half to one inch in 

 diameter, are also decomposed basalts, and still shew the felted 

 structure of the felspars very distinctly. Some of the whitish-grey 

 fragments are more finely crystalline than the basalt lapilli. They 

 consist of minute grains of greenish-grey kaolin interspersed with 

 dust of limonite. The fragments are probably felsite or altered 

 claystone, as both these rocks are found to underlie the basalts in 

 the neighbourhood of tliis tuff. Under the microscope the base is 

 seen to consist of semi-ti-ansparent microscopic grains of kaolin, 

 most of which show double refraction. Some of them may pro- 

 bably be felspar, but none of them shewed evidence of twinning. 

 The kaolin is just in the condition observable in the partially 

 kaolinized portions of felspar crystals, the crystals breaking up 

 into a mosaic of kaolin. Fragments of limonite are abundant, and 

 dusty grains of the same mineral are tolerably plentiful in the 

 base, though not in such abundance as in the basalt or felsite 

 lapilli. Spherical concretions of limonite occur in the base 

 occasionally. 



In specimen No. 1 {h) the tuffaceous character of the rock is less 

 apparent owing to its more advanced state of decomposition, the 

 vase and the fragments being blended together. There is a large 

 amount of segregated limonite in this specimen. The limonite 

 occurs in minute spherical concretions, and delicate concentric 

 shells, alternating with ochreous volcanic dust. The limonite also 

 encrusts the fragments of decomposed basalt and forms irregular- 

 shaped shells, one-quarter to half an inch thick, round portions of 

 less decomposed volcanic dust. This iron has evidently been 

 derived from the free magnetite in the basalt, and from the com- 

 bined iron in their augite and olivine. 



