Hyndman and Bonnet/ — Analysis of Spherutites. 3G5 



possible that the crystals possess cores of albite surrounded by zones 

 of orthoclase. A striated felspar has been found in a slide prepared 

 from another rock from the same general locality. A few pheno- 

 crysts of green augite are present, and also one or two of hornblende. 

 The ground-mass is obscure, but it appears to be made up of 

 a smaller generation of similar felspars, augite prisms, and magnetite 

 with little or no glass. 



Only two other rocks remain for description. One from Vongi 

 Mountain, on an island south-east of New Georgia, is a porphyritic 

 olivine-basalt (F. 243) in which the minerals are somewhat altered, 

 so that the rock may possibly belong to the rather older group 

 referred by Mr. T. Davies to the diabases. A similar rock, but one 

 in which the presence of olivine cannot be absolutely demonstrated, 

 is that from Gizo Island (F. 245). The felspars are much decom- 

 posed, and the augite somewhat altered ; the rock is a porphyritic 

 basalt, which contains pseudomorphs after olivine or hypersthene — 

 which, I cannot say. 



III. — Summary. 



The general result of the investigation of this collection is to 

 show that New Georgia consists chiefly of volcanic rocks belonging 

 both to the andesitic and basaltic division, and probably of recent 

 date. At the edge of the island organic rocks have been deposited 

 in connection with the great barrier reef, and no organic rocks have 

 yet been detected at any considerable distance inland. 



The chief rocks from Tetapari Island and the Florida group are 

 made of organic material mixed with volcanic mud, the latter some- 

 times entirely excluding the former substance. Occasionally the 

 calcareous matter is leached out of a rock of this nature, leaving a 

 rotten-stone behind which can only be distinguished from a pure 

 volcanic mud by its open texture and its casts of organisms. 

 Another result of this action is the formation of tufas. On and near 

 Florida Island there also occurs a peculiar deposit of siliceous 

 material, made of the remains of sponge spicules and radiolarian tests. 



Guppy notes that the interstices of coral rock are filled in with 

 the relics of foramiuifera and other small calcareous organisms. 

 On Tetapari Island this foraminiferal material occurs in considerable 

 quantity between the coral blocks in the limestone of the shore. 



VIII. — Analysis of a Spherulite and the Matrix in a Natural 

 and an Artificial Eock. 



By H. H. F. Hyndman, B.Sc. ; with Notes on their Microscopic Structure by 

 Prof. T. G. Bonney, D.Sc, LL.D., F.R.S. 



THE differences in chemical composition between spherulites and 

 the matrices which contain them have been determined by 

 many analysts. Michel Levy, in a long and comprehensive paper, 1 

 quotes some former analyses by Delesse, 2 comparing them with his 

 own, which they closely resembled. Lagario 3 gives many analyses 



1 Bull. Soc. Geol. France, v, 3rd series, p. 232 (1877). 



2 Ann. des Mines, xvii, 4th series, p. 116 (1850). 



3 See Teall's " British Petrography," p. 397. 



