236 A NATURALIST IN THE PACIFIC chap. 



divided according to the absence or presence of a groundmass into 

 two sub-classes, the first referring to the plutonic rocks, which, 

 however, are not represented in my collection, the second com- 

 prising the augite-andesites which make up 40 per cent, of the 

 total. These andesites are again divided into four orders, according 

 as the groundmass presents parallel or non-parallel felspar-lathes, or 

 short and stout felspars (orthophyric), or displays a felsitic character. 

 The last two orders are practically unrepresented here, though many 

 examples of them are found amongst the more acid andesites. 

 Each order is then split up into three sub-orders depending on the 

 nature of the pyroxene of the groundmass, whether granular, 

 prismatic, or ophitic. 



Each sub-order is broken up into two sections, one displaying 

 plagioclase-phenocrysts, the other without them, or possessing very 

 few of them. The first section is divided into two genera, according 

 to the character of the plagioclase-phenocrysts, whether glassy or 

 opaque, the second genus often comprising rocks allied to the 

 porphyrites. The second or aphanitic section is subdivided into 

 two genera according to the character of the pyroxene-phenocrysts ; 

 in the one case they are macroporphyritic ; in the other they are 

 either small or absent. The genera are split into four species 

 according to the length of the felspar-lathes, a method which 

 readily separates out the doleritic rocks. In cases where the 

 materials are abundant, the genera have been first divided into 

 porphyritic and non-porphyritic sub-genera, based on the macro- 

 porphyritic or the micro-porphyritic character of the plagioclase- 

 phenocrysts, when present. The species can be also split into sub- 

 species, according to the degree of basicity of the rocks, as indicated 

 by the specific gravity. 



This method is fully worked out in the later pages and need 

 not be further described here. With abundant material from 

 different regions it appears to me that a ready mode is here afforded 

 of assigning to a rock its place in the scheme. In this way it would 

 be possible to follow the systematist in his method of comparing 

 plants and animals from different localities. To facilitate this end, 

 I have suggested in the synopsis the employment of abbreviations, 

 so that the description of the critical characters of a rock can be 

 condensed into a formula capable of easy interpretation. 



As an example of the use of these abbreviations I will take the 

 instance of a common form of augite-andesite which is represented 

 by the^ formula : — " Plag^ aug, matr, Jiu, gran, non-phen, parv, 'l-^ 

 mm." This is the formula for an aphanitic augite-andesite, and it 



