290 7?. A. Daly — Mechanics of Igneous Intrusion. 



Formation of compound magmas by abyssal assimilation. — 

 To return to the main line of argument. If abyssal assimila- 

 tion by a magma be a fact, that magma must become more and 

 more mixed with the products of assimilation. Any eruptions 

 during, and subsequent to, the mixing might be expected to 

 show indications of the gradual alterations of the magma in 

 the normal case where magma and invaded formations have 

 different chemical composition. The writer believes that the 

 sequence of eruptive-rock types derived from such a heterogene- 

 ous magma would not be directed simply by the assimilated pro- 

 duct, but would follow definite physical and chemical laws 

 which govern heterogeneous magmas however formed. These 

 laws are embodied in the modern principle of differentiation. 

 By differentiation, the irregularly constituted and compound 

 magma of assimilation becomes definitely split up into sub- 

 magmas which may, on eruption, represent actual igneous 

 rocks visible at the earth's surface. 



Differentiation of compound magmas. — Numerous recent 

 discussions and summaries of the well established facts on 

 which the doctrine of differentiation is based, render unneces- 

 sary in this paper a restatement of the evidence. No other prin- 

 ciple has yet been evolved which explains so satisfactorily the 

 phenomena of consanguinity ; the fact of type-constancy, in the 

 midst of variety, among the eruptive rocks of the world ; the 

 normal order of eruption ; the phenomena of complementary 

 dikes ; the border facies of many stocks ; and the existence of 

 basic and acid segregations. Michel Levy himself, so long a 

 powerful and adverse critic of the differentiation theory, has 

 of late been won over to a recognition of its value in explana- 

 tion.* 



The consequences of our hypothesis demand that some 

 emphasis be laid on the probable nature of the differentiating 

 process involved in the splitting up of the abyssal magma. 

 Without wishing to be understood as denying the probability 

 of several other causes for differentiation, the writer will note 

 those conditions which, in his opinion, favor one cause in this 

 particular instance rather than any other. 



Each year new facts are accumulated, going to prove the 

 existence, at no great depth below the earth's surface, of a 

 general ferromagnesian magma, and recent theories of the 

 igneous rocks have thus returned in one point to the older 

 views of Bunsen and others. As yet it cannot be affirmed that 

 this or any other subcrustal magma is normally solid by reason 

 of pressure and only locally fluid by the local release of the 

 pressure. Nor can it be certain that a rock fragment, stoped 

 out from the vault of an intrusive body, would sink necessarily 

 *Bull. Soc. G6ol. France (3), xxv, 326 (1897). 



