Geology and Mineralogy. 201 



kind occur among observed rocks of the crust, and it is not diffi- 

 cult to conceive that differentiation on a profounder scale in 

 the early periods of the earth's history could have produced the 

 crust of the present time from a magma like the above. 



The view of Merrill that the excess of iron and magnesium in 

 the older rocks is altogether the result of secondary accumulation 

 seems also to the writer open to question. If time is the only 

 factor necessary for the accumulation of metals and their com- 

 pounds, it would seem that all metals and their salts should show 

 preponderance in the early rocks. As is well known, this is not 

 the case. That the two metals most abundant in meteorites should 

 also be most abundant in the early rocks is prima facie evidence 

 in favor of the excess of these metals at that time. Even if the 

 Lake Superior iron deposits can be shown to be the result of sec- 

 ondary accumulation, it should be borne in mind that these are 

 but a part of the iron deposits of early periods. The great 

 bodies of magnetite of Laurentian age in the Adirondacks and 

 Canada appear to be of primary origin. Important iron deposits 

 in Norway are reported to be magmatic segregations of Archean 

 granite rocks. The great iron deposits of Sweden are connected 

 with early eruptive rocks, and so, too, are large ones of the Urals. 

 It seems doubtful, therefore, whether the excess of iron in the older 

 rocks can be altogether explained by the theory of secondary 

 accumulation. 



With regard to the origin of dolomite, it is probably true that 

 the view that it is formed by the gradual replacement of lime by 

 magnesia through the downward percolation of magnesian waters 

 is a generally accepted one, but it is hoped that this will 

 not prevent acceptance of another theory if satisfactory evi- 

 dence can be presented in behalf of it. Several students of the 

 subject have of late given excellent reasons for dissatisfaction 

 with the replacement theory, and other methods of origin are being 

 sought. One of the most complete of the recent summaries is by 

 Steidtmann,* who after an extensive study expresses the view that 

 " the occurrence of dolomites of vast thickness and extent cannot 

 find a ready explanation in the mutative agency of underground 

 waters." A similar view is expressed by Daly.f 



That the sea, and therefore the limestones formed in it, are 

 steadily becoming more calcic and less magnesian on account of 

 the greater solubility of lime salts, is clearly shown by Steidtmann 

 in the article mentioned. The highly magnesian nature of crinoid 

 tests, as shown by analyses,;}; seems to the writer a further indi- 

 cation of this change. In a more magnesian ocean such forms 

 could flourish in great abundance and become important rock- 

 forming agents, but as the amount of magnesia diminished they 

 gradually became extinct. 



*Jour. Geol., xix, 342, 1911. 

 t This Journal, (4), xxiii, 109, 1907. 



tH. W. Nichols, Pub. Field Col. Mus., Geol. Ser., iii, 49, 1906 ; Clarke, 

 Bull. 491, U. S. G. S., p. 540. 



