312 



and tertiary rocks were removed by the agitation of the waters, and the 

 whole covered with that mass of disruptured fragments of rocks of all 

 ages, which constitute our diluviums. 



2d. When the upheaving force became stationary, at long and succes- 

 sive intervals, or when the elevation of mountain ranges had erected 

 barriers against the influx of the ocean. During this period lake al- 

 luvions would be forming over the area then occupied by the waters. 



3d. The era of the present levels ; which commence after the increas- 

 ing pressure had burst passages successively through the opposing bar- 

 riers, occasioning sudden subsidences of the waters. 



Thus we may suppose that the lakes assumed their present forms in 

 successive order, beginning with the most elevated, until finding their 

 final passage through the St. Lawrence, the chain as now existing, be- 

 came complete. 



Thus, (whichever assumption be correct,) the " lake ridge " becomes 

 a record of one of those great changes which the surface of the world 

 has b^en ever undergoing, to fit for the habitation of man. 



Detroit, January 24, 1841. 

 To Douglass Houghton, State Geologist: 



Dear Sir — Immediately upon my return from the portion of our 

 State bordering on Lake Superior, where my services as Assistant, had 

 been required during a large part of the season, I re-commenced the 

 detailed surveys in the organized counties of Michigan proper. These 

 were conducted with a more especial view to the determination of the 

 extent and value of the coal district of the Peninsula. The counties in 

 which minute examinations have been made during the past sea- 

 son, are Barry, Clinton, Shiawassee, Genesee, Lapeer, St. Clair, and Ma- 

 comb ; and examinations have been extended generally over other coun- 

 ties previously examined, in part, in order to the more full and satisfac- 

 tory completion of the duties assigned me. 



MAPS OF the counties. 



The labor of correcting, while in the field, maps of those counties 

 which were assigned to my Geological and Topographical supervision, 

 has been completed, and the plats are now in the hands of the State 

 Topographer, to be prepared for publication. A great mass of infonna- 



