ORIGIN' OF THE DOLOMITES. 107 



limestones was 3.11 per cent. The difference in specific gravity between 

 water and ordinary compact dolomite would raise this to near 9 per cent 

 of actual hulk. If, in considering limestone and dolomite in the mass 

 aivl in molecules, bulk were the same, 9 per cent of the limestone con- 

 tents of these formations might be counted out in calculating the reduc- 

 tion in thickness of the beds. It is not necessary in this sunnnar}' to 

 j)resent tables of calculations. The flow of waters from the dolomitic 

 beds, with their load of calcium carbonate, can in time produce but one 

 result, and that is to bring into more equal proportions the quantity of 

 calcium and magnesium carbonates. The disappearance of over 80 per 

 cent more of the former than of the latter, which occurs if the original 

 rocks of these beds had the composition of average modern marine de- 

 posits,* must result in the removal of at least eight times as much of that 

 material as remains behind. Under such an assumption every 100 feet 

 of the present thickness of the Oneota and Shakopee would represent an 

 original thickness of 1,000 feet, more or less, an extent much nearer in 

 accord with what seems necessary in sedimentary accumulation to con- 

 form to the profound faunal changes and crustal movements so conclu- 

 sively i>roved l)v the paleontologic and structural conditions of the rocks. 

 Further, that much reduction has taken place in the sandstones and 

 arenaceous shales of the Magnesian series is shown in the remarkable 

 freedom of these rocks from the several impurities so universal in recent 

 rocks of this character. So far as erosion has disclosed them, the granitic 

 and (piartzitic rocks, the source of these sandstones, contained these im- 

 purities to as high an extent as the same rock species in any other region. 



BASIS OF THE DISCUSS I OX. 



In this discussion the authors have not attempted to advance any new 

 theory of the origin (jf dolomites and dolomitic limestones. The simple 

 question before them has been this : Cannot the great reduction in bulk, 

 the pronounced changes in chemical comj:)osition and physical characters 

 which the dolomites of thenigion studied have undergone, be ex])lained 

 through the operation of such forces and processes as are to be seen going 

 on at the present time? 



Summary. 



The .Magnesian series discussed in the foregoing pages consists of four 

 alternating formations of dolomites and sandstones belonging to the 

 Upper Cambrian and a fifth of dcjlomite which, on i)aleontologic grounds 

 when its fauna shall be studied, may bo considered a part of the Ordo- 

 vician. 



♦See ante, p. 102. 



