448 J. G. Goodchild — Granite Junction in Mull. 



rnacle a long time before geologists ia general were disposed to give 

 any serious attention to them. 



Some years after the review appeared Mr. C. T. Clough, of the 

 Geological Survey, after a long and careful series of observations in 

 the field, came to the conclusion that the Whin Sill of Teesdale — a 

 M'ell-known intrusive sheet of dolerite invading the Carboniferous 

 rocks of the North of England — replaced these rocks, and had not 

 wedged them asunder, as it had commonly been supposed they had 

 done. Mr. Clough's observation made it certain that the total 

 thickness comprised between any two given upper and lower 

 horizons in the strata, invaded by this dolerite remained, exactly the 

 same where the Whin Sill was present as it did where it was 

 absent, and that the strata above it and below remained, not only at 

 their normal distance apart, but were absolutely undisturbed by the 

 intrusive mass, whatever its thickness might happen to be. Mr. 

 Clough argued that eruptive rocks such as these had assimilated the 

 strata whose position they occupy at present ; and he further 

 accounted for the uniformity of composition of the dolerite whei-e 

 the rock invaded happened to present a wide range of chemical 

 composition by the theory that a kind of circulation went on 

 throughout the molten mass, whereby the rock material, as fast as it 

 was melted up, became diffused throughout the whole mass, instead 

 of remaining more or less localised. 



Facts of the same nature as those observed by Mr. Clough had 

 been repeatedly noted in the areas adjoining by his colleagues on 

 the Survey ; and, indeed, every competent geologist who had 

 examined the field-evidence was entirely in accord with Mr. Clough 

 in his conclusions. 



But the cabinet geologist declined even to consider the matter — 

 he said that it was simply impossible that an intrusive mass could 

 replace the rocks it invaded ; and he found so numerous a body to 

 agree with him, and therefore disposed to rest content with the old 

 view, that it was clear to those who held more advanced views that 

 there was no chance of the subject meeting with fair discussion for 

 a long time to come. So the matter was allowed to drop, and the 

 steady-going section of geologists again went on their way in peace. 



It is somewhat remarkable that there should be so much reluct- 

 ance to accept this view, considering that the best text-books had, 

 long before, given illustrations of this very mode of occui-rence of 

 intrusive rocks, and that the maps and sections published by the 

 Geological Survey furnished abundant evidence pointing to the same 

 conclusions. And it is almost inconceivable how anyone who had 

 worked out the inter-relations of these rocks in the field, could 

 possibly adopt any other view. Apart from any theoretical bias 

 that a field-geologist might have in this matter, he could not very 

 well overlook the fact that his fellow- worker, the miner, has long 

 been aware that trap rocks very often "cut out" the coal seams or 

 other strata with which they come into contact. This is abundantly 

 proved in the Midland coal fields, as well as in those of the more 

 northern parts of the kingdom, and it is a rule of very general 

 application elsewhere. 



