WHY THE ROCK IS A DOLOMITE 621 



for me by Mr. W. J. Mead, 20 and the result arrived at is 1.31 per cent. 

 This shows a porosity about one-fourth as great as the average of five 

 determinations from the Niagara dolomite of Wisconsin, 21 almost the 

 same as that of a typical dense limestone from the Trenton (Platteville) 

 formation of southern Wisconsin, and only about four times as great as 

 the average porosity of such extremely compact rocks as granite, rhyolite, 

 and diabase. This comparison seems to preclude the possibility that the 

 formation has been subject to leaching action on a considerable scale ; and 

 although the discrepancy between 1.31 and 4.7 is not great, it affords 

 some ground for suspecting that the dolomite is not a result of the sub- 

 stitution of magnesium for half of the calcium in a normal limestone. I 

 may further point out that the general question of the origin of dolomites 

 by the alteration of limestone has been recently discussed by Steidtmann, 22 

 with an adverse conclusion. A cogent objection to this hypothesis, pointed 

 out by Steidtmann and others, applies well to the Bighorn and other dolo- 

 mites of the Paleozoic section in Wyoming; they are underlain by and to 

 some extent even interstratified with beds of pure limestone. The sharp 

 boundaries of these unlike beds of pure limestone ". . . seem to be 

 sufficient proof that the magnesia has not traveled appreciably through 

 the mass of strata since it was originally deposited in the sediments. 7 ' 

 The same inference is to be made from two thin sections which show 

 parts of corals. In both cases the coral thecse and septa are wholly dolo- 

 mitized, but some of the chambers are lined with calcite crystals. These 

 vug fillings are clearly secondary, and the relations indicate that calcite 

 has been deposited from solution while the dolomite remained fixed. 



The remaining (fourth) suggestion to be examined is that the material 

 of the Bighorn dolomite was originally deposited in the form of lime car- 

 bonate growths, shells, or ooze, but was progressively altered to dolomite 



20 The method of determination was as follows: After having been saturated with 

 water by immersion for five days the sample weighed in air 775.2 grams. When weighed 

 in water the result was 500.0 grams. It wns then dried Cor about 24 hours in an oven 

 at a temperature of about 200° centigrade. After this drying its weighl was 772 grams. 

 From these data it appears that the sample absorbed 3.2 grams of water. Its porosity 

 was then calculated by the following formula : 

 Weight dry 



:;.sir,. 



Weight in water saturated — Weight in air saturated 



Weight In air dry x 3.845 = 241 cc. = the volume of the rock material. 



3.2 grams = 3.2 cc. (approx.) = volume of water absorbed. 



3.2 



Porosity = - =.0131, or 1 31/100ths per cent p^ic Bpace. 



241 - 8.2 



21 Taken from "Building stones of Wisconsin," by ED. Et. Buckley. Wisconsin Geo 

 logical Survey, Bull. IV, 1898. 



M E. Steidtmann: The evolution of limestone and dolomite. Journal of Geology, vol, 

 xlx, 1911, pp. 825-828. 



XLIII— Bull. Geol. Soc. Am., Vol. 24, 1912 



