110 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 1 894, 



of composite dyke where the fissure has been reopened. In some 

 cases, he says, the acid rock was clearly introduced after the basic, in 

 others the order of ejection of the two materials was reversed. In 

 some instances the line of weakness, along which the opening and 

 re-injection of the dyke took place, lies towards the centre, at other 

 times it is at the side of the dyke, and occasionally it traverses the 

 dyke-mass in a sinuous manner. He consequently claims that the 

 interval between the first and second injection of these dykes must 

 have been sufficiently long to allow of the complete consolidation of 

 the older rock. As regards the respective ages of the acid and 

 basic rocks in these dykes, if position has anything to do with 

 it, I would point out that, out of five diagrammatic plans of these 

 dykes or of portions of them, in every case, whether the containing 

 rock be granite or sandstone, the augite-andesite or basic material 

 is represented as lining the sides, whilst a centre of varying width 

 is occupied by quartz-felsite, pitchstone-porphyry, pitchstone, dacite, 

 or some such rock. 



In his summary, it will be observed that the Author especially 

 guards against any suggestion as to the accidental association of 

 the augite-andesite and ' pitchstone ' in these composite dykes 

 of Arran. All the facts, he says, point to the conclusion that the 

 fissures were injected from the same subterranean reservoir, but that 

 this reservoir contained two magmas of totally different chemical 

 composition. Rejecting in the case of the dykes of Arran the idea 

 of selective crystallization and liquation in the already injected 

 materia], he says that we are compelled to fall back upon the view 

 that an actual separation takes place amongst the materials of a 

 molten magma before the work of crystallization has commenced. 

 How this may have been effected has exercised the minds of experi- 

 mentalists and others for a long period of time. 



Before quitting this part of Scotland I must draw your attention, 

 though very briefly, to another matter in connexion with these 

 Tertiary volcanic rocks. Prof. Judd, who had the good fortune to 

 make some of the most remarkable discoveries in the "Western Isles 

 of Scotland which have been placed to the score of a British 

 geologist, has long been at issue, as you all well know, with an 

 equally distinguished authority as to the order of appearance of 

 certain Tertiary eruptives. Quite lately, in speaking of the five 

 great centres of volcanic outburst in the "West of Scotland, Prof. 

 Judd again affirmed the sequence in time of the three kinds of 

 igneous material to have been, firstly the rocks of intermediate 



