282 A NATURALIST IN THE PACIFIC chap. 



banded appearance of some of the rocks arises from the glass 

 collecting in streaks rudely parallel and running in the direction of 

 the " flow " of the felspar-lathes. 



The only specimen in my collection of " sliced " rocks belonging 

 to the less basic sub-species is an altered bluish-grey rock (sp. gr. 

 27) from the range between the Mbuthai-sau valley and the 

 Wainikoro plains. Its long parallel untwinned felspar-lathes give 

 the nearly straight extinctions of oligoclase. Fine cracks in the 

 rock are filled with crystalline silica. 



Species C, felspar-lathes - 2 — -3 mm. long. 



Species D, „ „ -3— -5 ., 

 The rocks of these species in the collection are for the most part 

 dyke-rocks of the more basic kind. They are blackish or dark- 

 brown, almost doleritic in texture, and range in specific gravity 

 from 277 to 2*87. At times they are vesicular or scoriaceous, as in 

 the specimens from an agglomerate at Undu Point and from a flow 

 or dyke at Vunikondi. The most typical of these rocks are those 

 of some of the dykes of the Ndriti basin, which, however, display 

 propylitic alteration in a varying degree. They would be 

 described as semi-doleritic basalts without olivine or as non-porphy- 

 ritic basaltic andesites. Plagioclase phenocrysts are typically 

 absent, or they are scanty and not over 1 mm. in size. Augite 

 phenocrysts are usually scanty and small. The felspar-lathes, 

 which are more or less in flow-arrangement, are rather stout, and 

 range in average length in different rocks from '23 to "35 mm. 

 They often show a few twin-lamellae which yield extinctions of 

 medium and basic andesine (15 — 28 ). The augite granules are 

 large ('03 mm.) in the Ndriti rocks. Magnetites, usually corres- 

 ponding in size to the augite granules, are abundant. Interstitial 

 glass occurs often in fair quantity and is dark and semi-opaque. 



At times there can be recognised a later generation of minute 

 felspar microliths between the much larger lathes. They display 

 a plexus rather than a flow-arrangement. Whilst the larger 

 parallel lathes of the Vunikondi rock, above referred to, average 

 •23 mm. long, the felspar microliths of the interspaces average only 

 •03 mm. The significance of these two crops of felspars in the 

 groundmass is discussed on page 237. 



The only rock of the less basic sub-species in my collection is 

 from a dyke near Vatua-karoa. It shows secondary calcite and 

 viridite and other evidences of the propylitic change. The 

 felspar-lathes, which average "3 mm. in length, give extinctions of 

 oligoclase (o — 5 ) The specific gravity is 272. 



