C— GEOLOGY. 53 



They may be formed of portions of older ring-dykes or other intrusive 

 masses, lavas, or pre-Tertiary sediments. They usually show a fairly 

 high state of metamorphism for they have come under metamorphosing 

 influences from both sides. Igneous rocks in screens generally show 

 Tecrystallization and granulitization, while sediments have usually been 

 hornfelsed in a manner dependent on their respective original composition. 



Such masses locally interpolated between two intrusions that elsewhere 

 have reacted with each other at their junction, will at once free us from 

 doubt : firstly, as to the separate nature of the intrusions, and secondly, as 

 to their relative ages. , . 



In Mull, screens were found to be of the utmost value in determining 

 the relations of the intrusive rock masses to each other, and the same 

 has proved to be the case to a like degree in the ring-dyke complex of 

 Ardnamurchan. 



Turning now to a consideration of the composition and origin of the 

 major intrusions, the fact that impresses us most is the remarkable and 

 more or less constant association of widely divergent rock types. All 

 through the Tertiary province of Britain and Iceland gabbros and grano- 

 phyres or granites are the dominant plutonic intrusions, and may be said 

 to characterise the province as a whole. There can be no question that 

 these types are more or less extreme differentiation products of a common 

 magma, and this magma we may reasonably infer was that which supplied 

 the plateau basalts. Except in the case of certain cone-sheets, dykes and 

 other minor intrusions, this magma in an unmodified state is unrepresented 

 amongst the Tertiary intrusions. All the plutonics, as well as the majority 

 of the cone-sheets and sills, are products of a magma that must have 

 substantially changed its composition. In other words, almost every rock 

 type, excepting those specially mentioned, is a product of differentiation. 

 This fact leads us to the unavoidable conclusion that differentiation had 

 at any rate commenced in the local intercrustal reservoirs beneath the 

 plutonic centres before any intrusion into the upper part of the crust was 



effected. 



The gabbros and granophyres often occur, either alone or together, 

 without any rocks of intermediate composition, but these latter are 

 occasionally well represented, as in Mull and Ardnamurchan, by minor 

 intrusions such as sills and cone-sheets and occasional larger masses. Taken 

 as a whole the rocks of intermediate composition are of considerable bulk, 

 and therefore their corresponding magma should be given an honourable 

 place in any scheme of differentiation. As to the manner in which 

 differentiation was effected, we are convinced that the differentiation of a 

 normal magma cannot be accomplished by any process that does not 

 depend primarily upon the separation from it of solid crystalline phases. 

 Recent work both in the field and in the laboratory demonstrates that 

 there is no necessity to call to our aid such theories as those of dual magmas, 

 immiscible liquid fractions, or magmatic assimilation to account for the 

 differences of composition displayed by the major intrusions. 



In all the Tertiary centres the plutonic masses show but the feeblest 

 attempt at differentiation in place, and thus it is clear, with one notable 

 exception, that such variation of rock type as is met with was determined 

 before the respective magmas came to occupy their present positions. 



