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4 



on the Lake Superior Land District. 233 



and the largest 250 feet in height. The same effects would take 

 place in the depths of an oceanj only far more vast, when the 

 opened fissures lie their whole length exposed to the waters; 

 and the results would vary according to the condition or progress 

 of the eruptionj the currents that were in action at the time, and 

 the character of the region around. Adding also the friction of 

 the eruptive rock against the walls of the opened fissures, as sug- 

 gested by von Buch, and we have a sufficient cause for the forma- 

 tion of the fragmentary beds. 



That there were actual volcanoes in the Lake Superior region, 

 may be doubted ; for the same results as are there observed might 

 take place by eruptions through fissures without the existence of 

 a permanent vent and cone. In fact, nearly all (if not all) the 

 ejections about modern volcanoes are through fissures in the 

 slopes of the mountain and in the surrounding country. Farther 

 evidence is needed on this point. 



The sandstone of the region contains beds of the conglome- 

 rate and its layers alternate in many cases with the beds of trap, 

 proving that all belong to one epoch. The rock varies in color 

 from different dark brown shades to reddish or red; and also 

 passes into a slate. 



The compact or lower magnesian limestone contains fossils, 

 ^vhich are shown by Mr, James Hall to correspond in age with 

 the lower of the New York series. A conspicuous fossil is a 

 Maclurea like a species from the Chazy limestone. The Lep- 

 tasn^ have the characters of L. sericea, which in its highest range 

 does not extend above the Clinton group of New York. The 

 species of Ortfiis^ though too obscure to be identified, have the 

 characters known only in those of the lower Silurian beds. A 

 Crinoid {Gli/piocrinus) is not more recent than the Hudson 

 River group. The other fossils observed tend to the same con- 

 clusion. 



The age of the conglomerate associated with the trap is the 

 disputed point to which we have referred in the early part of 

 this review. The resemblance of the trap ranges and the asso- 

 ciated sandstones of Michigan to those of Connecticut and New 

 Jersey had led geologists to refer all to the same era; and the 

 Qge of the latter being shown by the occurrence of fossils, and 

 especially the fossil fish, described by Mr. W. C. Redfield, to be 

 subsequent to the coal, the era of the former was thereby supposed 

 to be settled. Dr. Jackson, after his examinations still sustains 

 this view. Messrs. Foster and Whitney state that the magne- 

 sian limestone, whose age is evidently lower Silurian, has been 

 found by them to overlie the conglomerate, and hence they place 

 the trap and conglomerate in the lowest Silurian or as an equiva- 



Secx)nd Series, Vol XII, No. 35.— Sept., 1851. 30 



