310 J. D. Dana on American Geological History. 



As plants may live in water too hot or impure for animals, 

 and moreover, since all nature exemplifies the principle that the 

 earth's surface was occupied with life as soon as fitted, and with 

 the highest forms the conditions of the time allowed, we may 

 reasonably infer that there may have been in Azoic times ma- 

 rine species and plant-infusoria, forms adapted to aid in the 

 earth's physical history; and thus vegetation may have long 

 preceded animal life on the globe.* 



After these general remarks on the divisions of Geological 

 time, I now propose to take up the characteristic features and 

 succession of events in American Geology. 



Report for 1846, 1847, and that for 1848, he points out several examples of the 

 Silurian covering the contorted Azoic, and his subsequent surveys have added to the 

 facts of this kind. They occur north of the lakes Huron and Superior, and along 

 and to the north of the St. Lawrence . Moreover in the vicinity of the lakes just men- 

 tioned, he found the Azoic divided into two unconformable groups, a lower, since 

 called by him the Laurentian, and an upper, the Huronian ; the former consisting 

 of granite, syenite, gneiss, hornblende rock, hypersthene rock, crystalline limestones, 

 etc.; the latter of diorite, slates, white and red sandstones, conglomerates, lime- 

 stones, the whole much intersected by trap and metalliferous veins containing native 

 copper, &c. t and having a thickness in some places, probably of 9000 to 12000 feet. 



Sections representing the nearly horizontal Lower Silurian overlying the Azoic, as 

 observed by him in the vicinity of the St. Lawrence northeast of Lake Champlain, 

 are figured in the Quarterly Journal of the Geological Society of London, for 1852, 

 pp. 203 and 206. 



In the progress of the Geological Survey of Hew York, commencing in 1836, the 

 fact that the crystalline rocks of Northern New York were older than the Silurian 

 was early shown, but good sections illustrating the superpositions of the two were 

 not given. 



At the meeting of the American Association at Cincinnati in 1851, when Foster 

 and Whitney first presented their views on the Azoic, Prof. Mather stated that he 

 had traced the continuation of the system nearly to the sources of the Mississippi 

 and on the waters of the St. Peters, — a region since reported on by Dr. D. D. Owen, 

 (Geol. Survey of Wisconsin, Iowa and Minnesota, 4to, 1852); Dr. H. King contribu- 

 ted observations on the Azoic or iron-mountain region of Missouri, (p. 194, Amer. 

 Assoc. Rep. 1851,) indicating the inferiority in position of these rocks to the Silurian, 

 as had been urged by Messrs. Foster and Whitney from the investigations by Mr. 

 Mersch under their direction ; and Dr. Engelmann described related rocks in Arkan- 

 sas between Little Rock and the Hot Springs. 



i Professors W. B. and H. D. Rogers refer to Azoic Rocks as found in the Appa- 

 lachians ; but no instances of the superposition of the lowest Silurian in those re- 

 gions on other non-conformable beds have yet been published ; and it is a question 

 whether the metamorphic rocks are all related to those of New England in age, or 

 partly of this era of metamorphism and partly Azoic. 



* The evidence with respect to the existence of plants in the Azoic Age, though 

 by no means positive, is stronger than here stated. — In the first place, there are 

 limestones among the folded strata ; and as limestones of later ages were almost 

 wholly of organic origin, these of Azoic rocks may also have been so. — 2nd, Graphite 

 is a common mineral in some of the crystalline rocks, and graphite is known to 

 result from the alteration by heat of the carbon of plants.— 3d, The Huronian rocks, 

 according to Mr. Logan, actually contain some small seams of anthracite. — 4th, 

 Vegetation, as it is directly or indirectly the food of animals, should necessarily have 

 preceded animal life.-- With reference to the statement in the text above, it should 

 be noted that vegetation has been observed growing among the Geysirs of Iceland, 

 in waters having a temperature of 180° F. ; and the writer has seen a case of similar 

 kind, on Luzon, one of the Philippines, where the temperature was 160° F. This is 

 much beyond the limit, which the eggs of animals can endure and survive. 



