xvn CLASSIFICATION OF VOLCANIC ROCKS 237 



signifies that it is a rock of the plagioclase-augite class possessing 

 a groundmass showing parallel felspar-lathes (between *i and 

 '2 mm, in average length) and granular pyroxene, and displaying 

 no phenocrysts of plagioclase or only a very few of small size, 

 whilst pyroxene phenocrysts if present are micro-porphyritic. 



As another example the following formula for a type of 

 porphyrite found in this island may be given : Plag, hypersth-aug + 

 matr, orth, prism, phen, opac. This is an andesite in which rhombic 

 and monoclinic pyroxene are associated both in the phenocrysts 

 and in the groundmass where the pyroxene is prismatic and not 

 granular. The plagioclase phenocrysts are opaque and the felspars 

 of the groundmass are of the orthophyric type. 



There are one or two points that require further reference. In 

 the first place the early employment in the scheme of the characters 

 of the felspars of the groundmass for distinguishing the orders 

 scarcely seems needed in the cases where they take the lathe-form ; 

 but the importance of its early use is shown in the acid andesites 

 where it is certainly of prime importance in an early stage of the 

 classification to distinguish the rocks by the character of the 

 felspars of the groundmass, whether lathe-like, orthophyric, or 

 felsitic. It may also be objected that the two orders obtained by 

 dividing the lathe group into " parallel " and " non-parallel " 

 divisions are not equivalents of the two other orders, the ortho- 

 phyric and the felsitic. The distinction, however, between the flow 

 or non-flow arrangement, though in practice not always readily 

 established, is a far-reaching one. On a priori grounds the first 

 division might be expected to have no plutonic equivalent ; whilst 

 in the second division, it is easy to trace the gradations through the 

 doleritic stage, where the felspar-lathes are very large, to the 

 granitoid condition. Then, again, the ophitic habit is as a general 

 rule confined to rocks with a doleritic or semi-doleritic groundmass, 

 where the felspar-lathes are coarse and form a mesh-work. Two 

 quite distinct lines of development unite in the felspar-lathes and 

 begin to diverge with the difference in their arrangement in the 

 groundmass. 



The nature of the difference between the flow and non-flow 

 arrangement of the felspar-lathes is well brought out in some dykes 

 of basalt and augite-andesite that I examined in this island and 

 also in the Valle del Bove on the Etna slopes. In the outer vitreous 

 portion the felspar-lathes, which are fairly well represented, are all 

 about the same length and are more or less parallel with the sides 

 of the dyke. In the central more crystalline portion two sets of 



