204 Proceedings of the Royal Physical Society. 



and these rocks happen to be invaded along the bedding 

 places by an intrusive sheet of dolerite 50 feet in thickness, 

 the two coal-seams are not found in such a case to be 150 feet 

 apart, as might have been expected to be the case, but are 

 seen to remain 100 feet apart, just as if the newer rock had 

 not been introduced at all. Even in hand specimens the 

 same fact maybe studied. I have lately presented one such, 

 from Aberdour, in Fife, to the Edinburgh Museum; and 

 many others showing the same feature can be observed in 

 the field, on all scales of magnitude. Moreover, it is a fact 

 well known to coal-miners that " sills cut out the coal-seams." 

 Geologists also have long been familiar with the fact that 

 granite and diorite bosses do not thrust aside the rocks they 

 invade, but act as if they had simply eaten their way up 

 through them. These are facts, explain them how one may. 



On the other hand, there is the equally well-known fact 

 that eruptive rocks do not vary in composition with litho- 

 logical changes in the rocks with which they are in contact ; 

 but in many cases they retain a marvellous uniformity of 

 both composition and structure, over hundreds of square 

 miles, and are the same in composition where they invade 

 limestone, as where they come into shales or into masses of 

 sandstone, and have no higher percentage of lime silicates, or 

 of compounds of alumina, or of silica, in each of these three 

 respective cases than in any others. (It may be remarked 

 that these notes are being penned in North Westmorland, 

 close to the outcrop of the Whin Sill, which exemplifies all 

 these points admirably.) 



Several years ago Mr Clough read a paper before the 

 Geological Society of London, in which he advocated the 

 view that in such cases as that of the Whin Sill the eruptive 

 rock had assimilated the rock of sedimentary origin, and he 

 endeavoured to show that, taking the bulk-analysis of these 

 sedimentary rocks over the whole of the area where the 

 intrusive rock is seen, that it would be possible to explain 

 the facts by assuming that the composition of the whole mass 

 of these would, if equalised, suffice to furnish the materials 

 which analysis showed to be present in the Whin Sill itself. 

 The view did not then meet with a very favourable reception 



