214 SCIENCE PROGRESS. 



" assimilation " as a factor of importance in the making of 

 plutonic or other igneous rocks in general. Any consider- 

 able incorporation of quartzites or grits should give rise to 

 an igneous rock of much more acid composition than any 

 with which we are acquainted, and similarly the taking up 

 of calcareous rocks should show itself in an abnormal content 

 of lime. As regards the latter, Brogger (11) takes the 

 instance of a large granite intrusion among the Lower 

 Palaeozoic formations of Southern Norway. The strata, 

 which abut on the granite, and are truncated by it, are 

 accurately known ; most of them are calcareous, and it is 

 calculated that the mixture obtained by melting them to- 

 gether in the proper proportions would have 24^- per cent. 

 of lime. The granite, however, contains only ^ per cent. 

 This is not too large an amount for an ordinary granite ; 

 but even assuming the original magma to have been free 

 from lime, it is seen that it cannot have taken up more than 

 one forty-eighth part of its own mass from rocks like those 

 seen in contact with it. Brogger asks, what then has be- 

 come of the prolongation of the strata cut off by the granite? 

 Since the portions cut off have not been incorporated in the 

 magma, they must exist beneath or beyond the granite. In 

 other words the, granitic body must have a concealed under 

 surface, and be an irregular sheet or lens. The argument 

 is evidently one not restricted to the particular case dis- 

 cussed. 



We shall conclude these notes on the morphological 

 characters of igneous rock-bodies by drawing attention to 

 certain peculiarities well exhibited by the Tertiary dykes and 

 sills in the Western Islands of Scotland (13, 21). 



One noticeable feature is the tendency of these igneous 

 intrusions to follow lines already marked out by earlier in- 

 trusions of like material, and probably from the same source. 

 In this way arise multiple dykes and sills. Many basalt 

 dykes in the volcanic area of Skye, for instance, are not 

 simple bodies corresponding in each case with a single in- 

 jection of the basic magma, but are double, triple or perhaps 

 tenfold, consisting of several distinct dykes in juxtaposition 

 and representing as many successive injections. Not in- 



