THE EVOLUTION OF IGNEOUS ROCKS. 157 



examining the distribution of the eruptive rocks of greater 

 geological antiquity in Europe in relation to the Hercynian 

 and older mountain ranees. 



Although the fact and even some of the laws of differen- 

 tiation in subterranean igneous magmas might be inferred 

 with considerable probability from a comparison of lavas at 

 the same and at different volcanic centres, the phenomena 

 and possible causes of such differentiation are to be studied 

 especially in rocks which have consolidated under deep- 

 seated conditions, and have been exposed by erosion. Here 

 we find in close association rock-types of widely different 

 chemical composition, but often having peculiarities in 

 common which point unmistakably to their consanguinity, 

 and the relative ages of such associated rocks is indicated 

 by the manner in which the newer cuts through the older. 

 Further, we find not infrequently in one and the same 

 plutonic body of rock petrographical types differing chemi- 

 cally as well as mineralogically, but passing into one another 

 by imperceptible gradations. 



An excellent example of such a complex of plutonic 

 rocks is that described by Dakyns and Teall 6 in the Garabal 

 Hill district, near Loch Lomond. The major part of this 

 area of igneous rocks is occupied by a porphyritic granite. 

 To the east of this occur a non-porphyritic granite and a less 

 acid rock, described as tonalite or quartz-diorite, and then a 

 diorite proper, forming the margin of the complex, especially 

 on its south-eastern edge. Associated with this are patches 

 of ultrabasic rocks (peridotites). The complex thus includes 

 rocks ranging from thoroughly acid to ultrabasic, related to 

 one another in a definite manner. In some places a sharp 

 line can be drawn between adjacent rock-types, and when 

 this is so, the more acid rock is alwavs found to be newer 

 than the more basiz ; in other places the several types pass 

 gradually into one another. The several magmas were, 

 therefore, introduced in order, beginning with the most 

 basic and ending with the most acid ; but the process was 

 in a broad sense a continuous one, and it is irresistibly 

 suggested that the different magmas came from a common 

 source. A series of seven chemical analyses shows in the 



