Conglomerates in Gneit>sic Terranes — A.WincheU. 157 
composed of lamellar augite and feldspar. This conglomerate 
therefore, dijffers from the Ogishke conglomerate of north-east- 
ern Minnesota both ia the mineral character of the pebbles and 
in the nature of the groundmass. 
Though the list of pebbles differs somewhat from that cited 
from Seagull lake, the general resemblance is noteworthy. 
The dark pebbles elsewhere scattered through the gneiss of 
Saganaga lake are also very similar in character; and the evi- 
dence is quite clear that the pebble-supply of all parts of the 
region has had a common origin. 
The presence of pebbles so widely disseminated through the 
gneiss reveals this great Laurentian terrane in quite a new 
aspect. This character seems especially adapted to awaken re- 
tiections in the minds of those who hold to the theory of a 
purely igneous history for the crystalline rock-masses. No 
other origin for rounded pebbles possesses any plausibility in 
comparison Avith shore action. Such pebbles are everywhere 
regarded as evidence of fragmental accumulation. The great 
Ogishke conglomerate, whose borders are not over fifteen miles 
distant, is stocked with similar pebbles; and no one could enter- 
tain other theory respecting them than that of slow fashioning 
along an ancient shore. The prima facie evidence in reference 
to the Saganaga pebbles is entirely in favor of a similar origin. 
f shall hold it as incontestable that these pebbles are due to 
attrition along a shore. 
I do not forget the dictum of Von Buch in reference to the 
<^ruptive origin of certain conglomerates,* nor the application 
made of the principle by the founders of the "Azoic System,'' 
to the well known conglomerates of the cupriferous region of 
Kewenaw Point.f But the pebbles in the latter case are asso- 
ciated with amygdaloids of unquestionably eruptive origin; and 
moreover, they are alleged to consist chiefly of rocky material 
of the same nature. In both respects the Saganaga pebbles 
differ. They are not pebbles of the contiguous rock, and it is 
inconceivable that they have become rounded by friction during 
projection through it while in a molten state, or by contact 
*Von Buch, Geognostische Brief e, pp. 75-82. 
tFoster and Whitney, Rep&rt on the Lake Superior Land District, 1850, 
pp. 69-200; and ^mer. /o«r. Sci., II, xvii, 1854, pp. 11-38, 181-104. The 
same view was advanced by Houghton in 1841. 
