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, flu, gran, non-phen, parv, ·1-·2 mm._” This is the formula for an aphanitic augite-andesite, and it signifies that it is a rock of the plagioclase-augite class possessing a groundmass showing parallel felspar-lathes (between ·1 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 orthophyric 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 _à 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.