262 A NATURALIST IN THE PACIFIC chap. 



33. Genus of the Olivine-Basalts 

 Formula. — Oliv, matVy pauc^ non-flu, oph,phen, vitr. 



Characters. — Olivine scanty. In the groundmass the fel- 

 spar-lathes are not in flow-arrangement and the pyroxene is 

 ophitic or semi-ophitic. Plagioclase phenocrysts glassy. 



Description. — These brownish-black rocks are all of the 

 semi-ophitic type. Although no ophitic " plates " occur in the 

 slide, the augites of the groundmass have no longer the granular 

 form, but are large, '08 — "i mm. in size, and tend to invest the 

 felspar-lathes. The specific gravity ranges from 278 to 2*86. As 

 in other of the ophitic and semi-ophitic rocks of this collection 

 (genera 9, 10, 12, of the augite-andesites), the large size of the 

 felspar-lathes (-2 — -3 mm. long) of the groundmass gives a doleritic 

 texture in the slide. In most of the other characters these rocks 

 approach those of genus 25 which possess felspar-lathes more than 

 •2 mm. in length. But they are more often semi-vitreous, and 

 display a considerable amount of dark smoky glass showing 

 numerous magnetite rods and skeletal crystals with fibrous devitri- 

 fication. The plagioclase phenocrysts, which vary much in size 

 in different rocks (in some 2 or 3 mm., in others 4 or 5 mm.), give 

 extinctions of andesine labradorite (20° — 35°). They are often 

 eroded and contain numerous large black inclusions of the magma. 

 The pyroxene phenocrysts, which are of pale-brown augite, often 

 have an aggregate-structure, having been formed in situ. 

 Others again consist of single crystals and have been much 

 affected by the magma. The olivine phenocrysts, which are at 

 times deeply eroded, are generally small and in part serpenti- 

 nised. 



37. Genus of the Olivine-Basalts 

 Formula. — OHv, matr, pauc, flu, gran, phen, vitr. 



Characters. — Olivine scanty. In the groundmass the fel- 

 spar-lathes are in flow-arrangement and the pyroxene is granular. 

 Glassy plagioclase phenocrysts. 



Description. — Brownish-black rocks which cannot be dis- 

 tinguished, except in the flow-arrangement of the felspars of the 

 groundmass, from those described under genus 25. Like them 

 they enter into the formation of the basaltic plains of Sarawanga 

 and Mbua and elsewhere. Most of the rocks of this genus group 

 themselves into one type where the felspar-lathes average in 



