xlviii PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



and shales, called by tlie surveyors the Kinderhook group, and de- 

 cidedly held to be the lowermost division of the subcarboniferous 

 formation, although ascribed by Prof. Hall to the Devonian. The 

 occurrence of these beds as the commencement of the Carboniferous 

 system gives rise to a theoretical view which, although indorsed by 

 our eminent associate Dr. Dawson, I cannot but regard as over- 

 strained if it be applied to the several systems of strata as at present 

 classified. " We have," says Prof. Worthen, " at the base a frag- 

 mentary series composed of sandstones and shales, the debris of pre- 

 existing formations, in the middle calcareous and highly fossiliferous 

 beds, representing the higher divisions of the subcarboniferous 

 series, and ending in the ascending scale with another fragmentary 

 series, comprising the sandstones and shales of the coal-measures." 

 No doubt, however, as Dr. Dawson observes, " this recurrence of 

 cycles deserves a more careful study as a means of settling the 

 sequence of oscillations of land and water in connexion with the 

 succession of life." 



The Devonian strata occur in three divisions of very moderate 

 thickness, but are underlain by massive limestones of the Upper and 

 Lower Silurian epoch, which are no less important for the physical 

 character they confer on the northern portion of the State, than for 

 the astonishing quantity of lead-ore produced from them within a 

 few years. Prof. Whitney, favourably known for a number of works 

 on analogous subjects, has contributed a detailed Report on these 

 Silurian limestones and their contents, which, from his former ex- 

 perience in Iowa and Wisconsin, he was specially fitted to prepare. 

 The siliceous strata which form the base of the system, the equiva- 

 lents of the Potsdam sandstone of New York, never rise to the sur- 

 face in Illinois ; but the next group above, the lower Magnesian 

 limestones, on the level of the " Calciferous Sandstone " of the New- 

 York Eeport, make their appearance in an arch or undulation of the 

 strata at La Salle. Over these follows a series, for about 150 feet 

 in thickness, of alternating calcareous and siliceous bands, and then 

 the important aggregate of beds called the Galena limestone, 250 to 

 275 feet thick, mostly a typical dolomite, yellowish grey in colour, 

 and weathering in fantastic forms, which confer a picturesque charm 

 on the narrow valley opening to the Mississippi. Its fossils, which 

 are abundant, place it, together with the relatively thin underlying 

 bed termed the ''blue limestone," on a parallel with the Trenton 

 group as described by Prof. James Hall. 



A band consisting chiefly of shales, the " Cincinnati group," of no 

 great thickness, separates the" Galena" from the *' Niagara" lime- 

 stone — again a powerful mass of dolomite, very similar in its htho- 

 logical character to the galena-zone, but so diiFerent in fossil con- 

 tents as to have been termed, even before the advent of the Survey, 

 the " Coralline and Pentamerus beds." Singularly destitute of any 

 trace of the mineral treasures which are scattered so lavishly in the 

 lower dolomite, this thick bank of rock plays otherwise a very pro- 

 minent part in the physical geography of a large portion of the 

 Western L^nited States, forming the highest points in Illinois and 



