Review of Recent Geolgical Literature. 3 1 5 
sediments yet more energetically than in the intrusive sheets, 
and at still greater depth, would yield a thoroughly granular acid 
rock as the product of that absorption with the consequent differ- 
entiation. This does not imply, of course, that all granites are of 
this origin, but it is quite possible that most intrusive granites are 
either of this origin or have been more or less modified through 
•assimilation. 
"The difficulty of discussing those questions is largely owing to 
the absence of accessible lower contacts in the average granite body. 
All the more valuable must be the information derived from intru- 
sive sills. The comparative variety of such rock relations as are 
described in this paper does not at all indicate the exceptional 
nature of the petrogenic events signalized in the Moyie, Pigeon 
Point or Sudbury intrusives. It is manifest that extensive assimi- 
lation and differentiation can only take place in sills when the sills 
are thick, well buried, and originally of high temperature. All 
these conditions apply to each case cited in the present paper. The 
phenomena described are relatively rare largely because thick basic 
sills cutting acid sediments are comparatively rare. 
"On the other hand, there are good reasons for believing that a 
sub-crustal gabbroid magma, actually or potentially fluid, is general 
all round the earth; and secondly, that the overlying solid rocks 
are, on the average, crystalline schists and sediments more acid 
than gabbro. Through local, though widespread and profound, as- 
similation of those acid terranes by the gabbro, accompanied and 
followed by differentiation, the batholitic granites may in large 
part have been derived.* True batholiths of gabbro are rare, per- 
haps because batholitic intrusion is always dependent on assimila- 
tion. 
"The argument necessarily extends still farther. It is not 
logical to restrict the assimilation-differentiation theory to the 
granites. The preparation of the magmas from which syenites and 
diorites, for example, have crystallized, may have been similarly 
affected by the local assimilation of special rock formations. The 
development of some of the anorthosites of the Canadian and Adir- 
ondack Archean was possibly conditioned on the digestion of part 
of the associated crystalline limestones by phi tonic magma. 
"The officers of the Minnesota Geological survey have shown 
that the same magma represented in the soda-granite and grano- 
phyre of Pigeon Point forms both dikes and amygdaloidal surface 
flows. t The assimilation-differentiation theory is evidently as ap- 
plicable to lavas as to intrusive bodies. But demonstration of the 
truth or error of the theory will doubtless be found in the study 
•of intrusive igneous bodies rather than in the study of volcanoes 
either ancient or modern. 
* Cf. 11. A. Daly, op. clt. 
t N. II. Winchell. Final report, .Minn. i;.-ui. Sur.. vol. 4, 1.S99, pp. 
519-22. 
