CHAPTER II. 



DEPOSITION, DISTURBANCE AND DENUDATION OF STRATA — GENERAL PHYSICAL 

 STRUCTURE OF THE NORTHWEST. 



The geological series in our State is very complete from the 

 horizon of the oldest known rocks, to the top of the Carbonifer- 

 ous System. From this point to the Glacial Drift, the formations 

 observed in other parts of the county are, as far as investiga- 

 tions have extended, entirely wanting. All that portion of the 

 Michigan series lying above the Niagara Group, is found within 

 the limits of the Lower Peninsula ; while the Niagara Group 

 and all rocks below, are confined to the Upper Peninsula and 

 the islands at the head of Lake Huron. 



The rocks of the Upper Peninsula not haviug as yet come 

 under the observation of the present survey, it is not deemed 

 necessary to refer to them at the present time, any further than 

 to show their connection with the geology of the contiguous 

 districts. 



In order to convey a clear idea of the superposition and lines 

 of outcrop of our different rocks, it will be desirable to offer a 

 few words on the general conformation of the strata of the 

 Northwest. Although the stratified rocks of the country suc- 

 ceed each other in regular ascending order, it must not be sup- 

 posed that these strata always occupy a horizontal position, 

 that they are necessarily continuous between distant points, or 

 that any given stratum is always actually overlain by those 

 strata which belong higher in the series. The sediments from 

 which these rocks were formed, were seldom deposited in per- 

 fectly horizontal sea bottoms, but to facilitate our explanation, 

 we may suppose that they were. We will suppose, also, that 

 one series of sediments was deposited upon another for the 

 space of many ages, and forming a thickness of several thou- 



