42 27ie American GeoUxjist. July, 1897 
Laurentiati, ii' it have any chronologic significance, should 
be applied only to the former of these, and the latter, being 
of a later date as to origin and position, should have some 
other characterization. For the purposes of this discussion, 
they will be distinguished simply as sedimentary Laurentian, 
and irruptive Laurentian. The writer has already called at- 
tention to the differences that mark the crystalline rocks* 
whether igneous or sedimentary, and to the probable causes 
of those differences, based on theoretical considerations, and 
has reached the conclusion that the basal Laurentian gneisses, 
whether of the igneous Laurentian or of the sedimentary, 
must have had a cause other than that of normal irruption 
from the interior of the earth. f In an earlier paper read be- 
fore the American Association for the Advancement of Sci- 
ence, t he also reached the conclusion, based on theoretical 
considerations, that "all acid eruptive rocks result from the 
hydro-thermal fusion of preexisting sedimentary strata em- 
braced in the super-crust of the earth." At the same time 
this conclusion was based on field observations made in north- 
eastern Minnesota. Since then Bayley has more carefully 
studied the rocks on Pigeon point and come to the conclusion 
that the acid rocks, at least of Pigeon point, have been de- 
rived from the fusion of the sedimentary strata concerned in 
the upheaval and metamorphism of the Animikie.§ 
I wish to call the attention of the Academy to certain field 
evidences that prove as certainly that the Laurentian igneous 
rocks are due to fusion of the crystalline schists, and further, 
that the same process which produced the crystalline schists 
from sedimentary strata, produced also, when carried to the 
end result, the igneous rock which, in the form of granite 
dikes and extensive bosses, invades and overwhelms them. 
It is due to Mr. A. C. Lawson that geologists have become 
convinced that the igneous Laurentian is later than the sedi- 
mentary Laurentian, chronologically, although, in many in- 
*The crystalline rocks: Some preliminary considerations as to their 
structures and origin. Twentieth Annual Report, Minnesota survey, 
p. L 
tOp. cit. p. 27. 
JSome thoughts on eruptive rocks, with special reference to those of 
Minnesota: A. A. A. S., Cleveland, 1888, p. 212. 
§The eruptive and sedimentary rocks of Pigeon point, and their con- 
tact phenomena, Bui. 109, U. S. Geol. Sur., 1893. 
