164 R. A. DALY THE EVOLUTION OF THE LIMESTONES 



Cretaceous, Tertiary, and Quaternary-Kecent limestones have been calcu- 

 lated, so as to show the average ratio of calcium to magnesium through- 

 out the series. The analyses were taken from the government survey 

 reports of Canada and the United States ; from Logan's "Geology of Can- 

 ada"; from the state survey reports of Arkansas, Indiana, Iowa, Ken- 

 tuck} r , Minnesota, Ohio, Pennsylvania, West Virginia, and Wisconsin; 

 from the reports of the Ontario Bureau of Mines ; from Firket's elaborate 

 paper on the limestones of Belgium, 10 and from the list of analyses sup- 

 plied for the report on the geology of the Cordillera at the forty-ninth 

 parallel of latitude. 



The selection is far from being as complete as it might be made, but it 

 is believed that enough analyses are represented to give a fairly accurate 

 idea of the variation of the ratio through geologic time. The number 

 of pre-Cambrian and Cambrian limestones averaged is, in both cases, low, 

 but includes nearly all that seemed to be available. The number of 

 the Tertiary and later limestones averaged is again low, but the labor of 

 searching for additional ones did not seem necessary, since it is well 

 known that these later limestones are usually very low in magnesium. 

 Lesley had already prepared a remarkable series of analyses (230) which 

 was intended to afford the average ratio for the Ordovician limestones of 

 Pennsylvania. This result could not, however, be safely used, inasmuch 

 as the whole series refers only to some 370 feet of beds out of several 

 thousand feet of the limestones locally developed, and at that represents 

 only a local phase of the Ordovician. 11 It has thus seemed better to use 

 the analyses derived from many Ordovician formations in Canada and the 

 United States. The ratio for the pre-Cambrian may be a little too high, 

 for the reason that thirty-three out of the sixty-one analyses selected were 

 taken from Miller's Bureau of Mines report on the limestones of Ontario, 

 in which there was some tendency to select limestones specially adapted 

 to lime burning. Excluding twelve analyses of specimens from limekiln 

 quarries in Ontario, the average ratio for the remaining pre-Cambrian 

 rocks is 3.61 : 1. 



The results of the compilation and calculation are given in table IV. 



"A. Firket: Annales Societe Geologique de Belgique, vol. 11, 1883, p. 221. 

 11 J. P. Lesley : Final Report of Pennsylvania Survey, vol. 1, 1892, p. 327. 



