FORMER VIEWS AS TO ORIGIN OF THE TRAPS AND AMYGDALOIDS. 9 
erally extend through the different folds; while others possess a structure like the 
cellular tissue of wood. We have no confidence in the vegetable origin of these mark- 
ings, nor have we any theory to offer in explanation. 
The same views are as strenuously maintained in other publications by 
the same authors,’ who were, however, preceded in them by Dr. Douglas 
Houghton. The latter speaks of the conglomerates under the name of “ trap 
tuff,” and states distinctly that they are made up almost exclusively of pebbles 
and bowlders of ‘greenstone and amygdaloidal trap.”* Even those conglom- 
erates in which there is evident more or less sandy material he speaks of as 
made up entirely of comminuted greenstone, while the sandstone and shale 
which with so great thickness form the upper part of the series on Kewee- 
naw Point, he considers to be of sedimentary origin, the material having 
been worn from pre-existing granitic rocks. 
So far as I have noticed, none of the other writers on Lake Superior 
geology have accepted these peculiar views as to the origin of these con- 
glomerates—-all regarding them as made up of water-worn pebbles detached 
from some pre-existing rock—and for the excellent reason that Dr. Hough- 
ton’s and Messrs. Foster and Whitney’s facts, as well as their theory, failed 
to stand closer study. The included pebbles are only in very subordinate 
quantity of “trap” or amygdaloid, being almost wholly of some sort of acid 
eruptive rock, i. e., felsite, quartziferous porphyry, quartzless porphyry, 
granitic porphyry, augite-syenite, or granite. The fundamental difference 
between such pebbles and the associated basic massive rocks is alone 
enough to overthrow the theory, even were there not other sufficient argu- 
ments against it. Further, the pebbles are just as plainly water-worn as 
those of any other conglomerates, though they may have in some cases had 
the polish removed by surface alteration. 
The “trap” and amygdaloid are by some writers considered as having 
had a common origin, but by others are separated. Under the generic term 
of “trap,” Messrs. Foster and Whitney included ‘‘greenstone, granular and 
amyedaloidal trap, basalt, etc.” and held to their origin in part as lava 
flows, contemporaneous with the deposition by ordinary sedimentation of 
1 Op. cit., pp. 99, 100. 
2Am. Jour. Sci. (2), XVII, 1854, pp. 11-38, 181-194. 
3 Joint Documents, Mich., 1841, pp. 472-607. 
