VI. 



The Sequence of Plutonic Rocks. 



THE fact that the lavas and ashes ejected in any one district vary 

 in composition in the course of geological time has attracted the 

 attention of those geologists who have devoted themselves to the 

 study of volcanic regions from very early times, and attempts have 

 been made to explain this variation by hypotheses as to the 

 distribution of igneous magmas beneath the superficial crust of the 

 globe. The chemical researches of Bunsen on the volcanic rocks of 

 Iceland in 1851, led him to the view that the variations in composition 

 could be explained by assuming the separate existence of two magmas, 

 to which he applied the terms normal-pyroxenic and normal- 

 trachytic. Any igneous rock would, on this theory, have the compo- 

 sition of one or other of the original magmas, or else an intermediate 

 composition due to an admixture of the two magmas in some 

 definite proportion. The mixture theory of Bunsen is probably not 

 correct as an explanation of the origin of igneous rocks, but that it 

 expresses with a very fair approximation to truth the variations in 

 composition of volcanic rocks belonging to the basalt-, andesite-, and 

 liparite-families has been established by hosts of analyses. 



The "theory of the two magmas " is, however, usually associated 

 with the name of Durocher, who, in his celebrated " Essai de 

 Petrologie Comparee," endeavoured to explain volcanic phenomena, 

 and especially the sequence of volcanic rocks, by reference to two 

 universal magmas which were supposed to form concentric spheres 

 beneath the solid crust, and to be kept apart by reason of their 

 difference in density. The upper and lighter magma was supposed 

 to be practically identical with Bunsen's normal-trachytic magma, 

 and the lower and heavier with his normal-pyroxenic magma. At 

 the time when the theory was proposed it was commonly believed 

 that the earlier eruptions were always of a trachytic, and the later of 

 a basaltic character. This Durocher explained by supposing that 

 the erupted material was at first drawn from the upper or trachytic 

 layer, and finally, after this had been locally exhausted, from the 

 under or basaltic layer. He recognised, however, that there were 

 many facts which the theory as above stated could not account for, 

 and these he endeavoured to explain by " liquation," by the 

 accession of sea-water, and by the alteration of the magmas in course 



