28o A NATURALIST IN THE PACIFIC chap. 



andesites, where the felspar-lathes are as a rule very small (under 

 •I mm. in length), as well as some of the doleritic types where 

 they are very large ("2 to '4 mm. long). In assigning a rock a 

 place in this genus some regard must be paid to its macroscopic 

 aspect as well as to the presence or absence of plagioclase 

 phenocrysts. In many cases two or three small phenocrysts may 

 be observed in a slide, under a millimetre in size ; but they do not 

 give a character to the naked-eye appearance of the rock, and 

 such rocks cannot be distinguished from others that do not display 

 them. 



These rocks range in specific gravity from 2*55 to 2'S$. This 

 large range is in the main concerned with different degrees of 

 basicity depending on the character of the plagioclase, the 

 relative abundance of the augite granules, &c. ; but it is also 

 connected with the amount of interstitial glass. The variety of 

 plagioclase ranges between oligoclase and andesine labradorite. 

 The fluidal structure is nearly always well-marked, and the closely 

 packed felspar-lathes have often the peculiar " felted " appearance 

 of many andesites. A little interstitial glass is present in most 

 rocks. 



Many, perhaps nearly all, of the rocks belong either to dykes 

 or to larger intrusive masses. All the four species indicated by the 

 length of the felspar-lathes are represented in my collection, especially 

 the two with smallest felspars. They may again be split up into 

 two sub-species according to the degree of basicity of the rocks. 



Species A. — Felspar-lathes between '02 mm. and "i mm. in 

 average length. 



(i) Mosi basic sub-species . . . Sp. gr. 275-2*85. Dark-brown 

 and dark-grey compact aphanitic rocks showing no plagioclase 

 phenocrysts to the eye. When a few of these phenocrysts 

 are present in a slide they are not usually much over i mm. in size, 

 and give extinctions of andesine labradorite (20° to 30°). Augite 

 phenocrysts are often absent, and when present are not over i mm. 

 in size and are as a rule scanty, occasionally affording a suspicion 

 of inter-growths with rhombic pyroxene. The felspar-lathes 

 which display marked flow-structure, vary in average length in 

 different rocks from "05 — '08 mm. Lamellar twinning is rare, the 

 extinctions being those of oligoclase andesine (10° to 20°). The 

 usual extinction, as measured from the long axis of the lathe, is 

 10° to 15° (medium andesine). The augite granules are small 

 (•01 — *02 mm.) and abundant. There is generally a little interstitial 

 glass with small magnetite. 



