Per cent. Felspar 1.7 Granite 0.09 0.25 0.58 0.68 Lava 1.21 1.8 Trachyte 0.30 0.66 Basalt 0.50 1.11 Porphyry 0.26 Marl 1.45 2.31 3.8 Calcareous stones 0.064 0.176 Dolomite 1.24 Lias chalk 1.39 Gneiss 0.18 0.78 1.51 Syenite 0.10 Dolerite 0.3 1.1 1.2 Diorite 0.5 0.69 CHAPTER VI. THE POSITION OF POTASH IN AGRICULTURE. We may, lastly, consider the position of _potash_ in agriculture, the only ash ingredient of the plant, in addition to phosphoric acid, which it is as a rule necessary to add as a manure. _Potash of less Importance than Phosphoric Acid._ It is of far less importance than phosphoric acid, from the fact of its much more abundant occurrence in the soil, as well as from the fact that under the ordinary conditions of agriculture, although removed from the soil in considerable quantities by crops, it finds its way back again in the farmyard manure; for it has not the same tendency to accumulate in large quantities in the grain or seed as we have seen to be the case with phosphoric acid. On this account straw contains a much greater proportion of potash than phosphoric acid, and hence farmyard manure may be regarded as fairly rich in potash. _Occurrence of Potash._ Of all sources of potash the ocean must be regarded as the chief. Millions and millions of tons are present in a state of solution in the salt water of the ocean.[126] Like phosphoric acid, its occurrence in the rocks forming the earth's crust may be said to be practically universal. Many of the commonly occurring rocks and minerals are extremely rich in it, and by their disintegration furnish large quantities to the soil. Some of these rocks contain it in such abundance that they have been tried as potash manures; and were other more valuable sources less available than they actually are, such a practice might well be recommended. A volcanic rock known as _palagonite_, and that most commonly occurring of all potash minerals--viz., felspar--have both been experimented with in this way with considerable success. _Felspar and other Potash Minerals._