3 M The American Geologist. November, 1905 
nomenon. Each interpretation of field facts has been phrased in 
terms of magmatic differentiation versus magmatic assimilation as 
explaining the eruptive rocks actually seen on the contacts dis- 
cussed. Nothing seems more probable, however, than that such 
rocks are often to be referred to the compound process of assimila- 
tion accompanied and followed by magmatic differentiation. The 
chemical composition of an intrusive rock at a contact of magmatic 
assimilation is thus not simply the direct product of digestion. It 
is the net result of re-arrangements brought about in the com- 
pound magma of assimilation. In the magma, intrusion currents, 
convection currents and the currents set up by the sinking or rising 
of xenoliths must take a part in destroying any simple relation be- 
tween the chemical constituents of the intrusive and the invaded 
formations. Still more effective may be the laws of differention 
in a magma made heterogeneous by the absorption of foreign mate- 
rial which is itself generally heterogeneous. The formation of 
eutectic compounds or mixtures, the development of density strati- 
fication, and other causes for the chemical and physical resorting 
of materials in the new magma, ought certainly to be regarded as 
of powerful effect in the same sense." 
The author mentions other defects in the arguments usually 
launched against assimilation, and concludes as follows: 
"In the foregoing discussion the secondary origin of some gran- 
ites has been deduced from the study of intrusive sills or sheets; 
but it is evidently by no means necessary that the igneous rock 
body should have the sill form. The wider and more important 
question is immediately at hand — does the assimilation-differentia- 
tion theory apply to truly abyssal contacts? Do the granites of 
stocks and batholiths sometimes originate in a manner similar or 
analogous to that just outlined for the sills? 
"The writer has briefly noted general reasons affording affirma- 
tive answers to these questions.* 
"Gabbro and granophyre are often characteristically associated 
at various localities in the British islands as in other parts of the 
world. t The field relations are not there so simple as in the 
case of the Moyie sill, for example, but otherwise the occurrence of 
many common features among all these rock associations suggests 
the possibility of extending the assimilation-differentiation theory 
to all the granophyres. Harker's excellent memoir on the gabbro 
and granophyre of the Carrock Fell district, England, shows re- 
markable parallels between 'laccolite rocks' and those of Minne- 
sota and Ontario.t 
"At Carrock Fell there is again a commonly recurring transi- 
tion from the granophyre to true granite, and again the granophyre 
is a peripheral phase. Still larger bodies of gabbro, digesting acid 
* Am. Jour. Sci., vol. 15, 1003, p. 269; vol. 16, 1903, p. 107. 
; See A. Geikie. Ancient volcanoes of Great Britain, 1S97. 
t Quart. Jour. Geol. Soc, vol. 50, 1S94. p. 311, and vol. 51, 1895, p. 12&. 
