J. G. Goodchild — Atigen-sfructure and Eriqjtive Rocks, etc. 25 



to an extreme, felspar in a crystalline form has begun to develop in 

 conspicuous quantities. In other words, a rock that was originally 

 deficient in potash is known to contain a considerable percentage 

 of that alkali after the rock has been subjected to dynamo- 

 metaraorphism. Now, as the presence of crytalline felspar is an 

 essential feature in gneiss as well as in granite and other rocks, it 

 becomes a matter of considerable importance to determine the source 

 of the potash and other alkalies in question. Unfortunately the 

 evidence available as yet is but scanty, and will hardly suffice for 

 more than a working hypothesis such as that here propounded : — 



When an eruptive rock, such as granite, is exposed to the action 

 of surface agencies, its constituents, as Mr. Teall remarks, eventually 

 became separated into an insoluble and a soluble set. The first 

 goes to make up sandstones and shales, etc., while, in a soluble form, 

 the alkalies are carried away by running water. Part of the alkaline 

 matter goes into the soil, whence some of it is re-extracted by plants. 

 "Whether the whole of it does so is certainly open to question. It 

 is far from unlikely that small percentages are carried downward 

 through the soil by percolating water, and are eventually returned 

 to the inner zones of the earth's crust, whence they originally started. 

 The remainder of the alkalies, not absorbed by the earth, is carried 

 out to sea, into which rivers are, and have always been, pouring 

 considerable quantities of both potash and soda, etc., which have 

 been set free from the surface of the land by weathering. What 

 becomes of all this alkaline matter? The sun's heat distils only 

 fresh water from the surface of the ocean, leaving its dissolved salts 

 behind. Unless there are some agencies at work using up the 

 alkalies as fast as they arrive, they must go on accumulating, 

 and the balance of nature eventually be disturbed in this matter. 

 AlgEe take up some of the potash ; but algee die in the water, and 

 so restore to the sea what they have originally taken from it. 

 Glauconite and other minerals containing potassium compounds may 

 use up another part. But even these remain where they were formed. 

 Is it not more likely that both potash and soda in small quantites 

 are slowly returning to the inner zones of the earth's crust by down- 

 ward percolation from the surface of the land and the bottom of the 

 sea ? The quantity may not be great at any given period ; but great 

 geological results more often ensue from comparatively feeble causes 

 acting through long periods of time than from those of a more 

 energetic character. 



Downward percolation from the surface of the land and the 

 bottom of the sea may not be the only source of the alkalies 

 required for the regeneration of felspars. At the high temperatures 

 prevailing where rocks are undergoing plutonic metamorphism, 

 what can be more likely than that emanations of both potassium and 

 sodium in some mobile form should arise from the metallic zones of 

 the earth's crust, and thence permeate the heated rocks within range 

 of their influence ? For several reasons I am disposed to regard 

 this as the chief source of the restored alkalies, and to consider such 

 intromissions an essential factor in the conversion of metamorphosed 



