7 o 



A NATURALIST IN THE PACIFIC chap. 



altered greenish rock which was crossed by a vertical dyke, 4 feet 

 thick, formed of a dark grey less altered rock. I have referred 

 these two propylites to two different genera of the augite-andesites, 

 the dyke-rock to genus 2, and the other to genus 4. In the case 

 of the dyke the rock is a little vesicular ; whilst in the other it is 

 densely charged with pyrites. Both have been subjected to the 

 same alteration ; but in a different degree ; and it would thus seem 

 that solfataric influences were here in operation before and after the 

 intrusion of the dyke. 



With reference to the characters of the alteration of these rocks 

 of the Ndriti basin, it may be remarked that where the change is 

 greatest the felspars of the groundmass are alone recognisable. 

 The plagioclase phenocrysts are quite disguised by alteration 

 products, and chlorite, viridite, epidote, calcite, pyrites, &c, occupy 

 much of the groundmass. Other rocks are less affected and in a 

 few the change is only slight. 



With regard to the prevailing types of the propylites of the 

 Ndriti Basin, it has already been observed that in most of them 

 the felspar-lathes of the groundmass are unusually large, the 

 average length being - 3 mm. From the rare occurrence of olivine 

 in some of the rocks that are but slightly changed, it is to 

 be inferred that most of them belong to the augite-andesites, 

 and might be termed doleritic basaltic andesites. But in other 

 respects they differ considerably, both as regards the presence 

 or absence of flow-arrangement of the felspar-lathes, and in 

 the occurrence and size of the plagioclase-phenocrysts, some 

 having large porphyritic crystals, others small phenocrysts, and 

 others none at all. Many of them contained a little interstitial 

 glass. In my classification of the augite-andesites they are 

 assigned to genera 2, 4, 16, &c, and additional particulars con- 

 cerning their characters are given in the description of those 

 genera. Judging from the average large size of the felspar-lathes 

 it may be held that, although in other features they often differ, 

 some of the general conditions under which they were produced 

 were the same. 



On the right bank of the Ndama river, opposite Ndriti, there is 

 a singular association of a vertical dyke of a bluish-grey basic 

 andesite with a reddish scoriaceous lava, apparently a flow. The 

 dyke is about 4 feet thick and runs N.W. and S.E., like the other 

 dykes of the basin, exhibiting also a rudely columnar structure 

 across its breadth. Where the two rocks are in contact, the dyke 

 has a vitreous border half an inch thick, and an offshoot of the 



