(a) Quartz porphyries. (b) Liparites. (c) Quartz-free porphyries. (d) Trachytes. (e) Phonolites. (f) Porphyrites. (g) Andesites. (h) Melaphyrs and augite porphyrites. (i) Basalts. (j) Tephrites and Basanites. (k) Picrite porphyrites. (l) Limburgites and augitites. (m) Leucite rocks. (n) Nepheline rocks. (o) Melilite rocks. It is, in most cases, impossible to state which of the above classes is of most importance from an agricultural standpoint, since, in the process of soil formation, both chemical and physical processes are involved, whereby the character of the resultant soil is so modified as to but remotely resemble its parent rock. In most soils, the prevailing constituent is but the least soluble one of the rock mass from which it was derived. Thus a limestone soil may contain upwards of ninety per cent of silica and alumina, while the original limestone itself may not have carried more than one or two per cent of these substances. Of course, if a rock mass contains none of the constituents essential to plant growth, its resultant soil must by itself alone be quite barren. It does not absolutely follow, however, that those rocks containing the highest percentages of valuable constituents will yield the most fertile soils, since much depends on the manner in which they have been formed, the amount of leaching, etc., they may have undergone. Nevertheless, the study of the composition of the rocks in their relation to soils, is an extremely interesting and by no means unimportant one. A comparative table of rock compositions is here given. It will be observed that there is a considerable range of variation even among rocks of the same class, a fact due to the varying abundance of their mineral constituents. The figures given are not those of actual analyses on any one particular rock, but are selected from a number of comparatively typical cases; and, it is thought, fairly well represent the composition of the class of rocks indicated. COMPOSITION OF ROCKS.—THE FIGURES INDICATE PARTS PER HUNDRED.