On the Genesis of Some Scottish Minerals. 205 



— perhaps because it was advanced before its time. The 

 chief objection put forward against its acceptance was the 

 very obvious one above referred to, that the Whin Sill did 

 not vary in composition with the rocks in which it occurred. 



The view here presented may be regarded as a modification 

 of Mr Clough's. It may be stated as follows : — 



If we examine the evidence afforded by the analyses of 

 eruptive rocks on the one hand, and of sedimentary rocks 

 on the other, it becomes evident that the two differ chiefly 

 in the percentage of alkalies present. In the case of the 

 eruptive rocks the quantity is relatively large, and is small 

 in the case of the others. On this account no mere rearrange- 

 ment of the constituents of any normal sediment would 

 suffice to convert it into any kind of eruptive rock ; but 

 if it is possible by any means to restore the alkalies which 

 epigene causes have removed, then, and only then, does the 

 reconversion become possible. Hence it becomes a question 

 of much interest and importance to inquire by what means 

 any such restoration of alkali can be rendered possible. 



There are two possible sources ; but, concerning them both, 

 it must be confessed that at present we can form little more 

 than a conjecture. One of these may be connected with 

 the uprise of heated waters from below, which, it is at least 

 conceivable, might bring up sufficient quantities to effect in 

 time the result in question. The other, upon which I am 

 disposed to lay very much more stress, is, in one sense, 

 epigene in origin. It is supposed to take place beneath the 

 floor of the ocean — mainly within that marginal region which 

 forms the Transitional Zone between the line of mean sea- 

 level and the seaward margin of the land, and which is now 

 generally recognised as the zone of chief terrestrial activity. 

 Alkaline carbonates are ceaselessly being transferred in 

 solution from the rocks of the land to the sea by the agency 

 of rivers; and that transference must always have been 

 in progress. Hence, unless there is some natural agent, 

 organic or inorganic, which is constantl}^ at work using up 

 these materials as fast as they arrive, in the same manner 

 as organisms use up the lime-salts, it is obvious that 

 the quantity must go on increasing, which we have no 



