G. P. Merrill— Minor Constituents of Meteorites. 525 



of earlier times was more nearly meteoric in composition than 

 the present seems to be indicated by the great deposits of iron 

 oxide of earlier ages and the fact that the early limestones are 

 more magnesian than the modern." That the older rocks con- 

 tain a larger proportion of these substances than the more 

 recent is unquestionable, but such are products of secondary 

 accumulation and cannot be accepted as proof of early abun- 

 dance. The greatest deposits of iron ore of which we have 

 knowledge are merely secondary or results of leaching of lean 

 ores in the older rocks and a concentration of much more recent 

 date. Whether we' accept the early views to the effect that the 

 Lake Superior ores were due wholly to leaching from lean, 

 cherty carbonates or greenalite beds and reconcentration, or the 

 more recent ideas of Van Hise and others,* that the main mass 

 of the iron-bearing sediments was derived from associated 

 eruptive rocks, we have to look for the original source of the 

 material to rocks not richer in iron than those of very recent 

 origin. Concentration has simply gone on for a longer period 

 and the results are naturally of greater magnitude. This is 

 recognized by Van Hise.f In discussing the origin of the ores' 

 of the Menominee district he says, " the process of ore concen- 

 tration was well advanced before Cambrian time." In the 

 Mesabi district " the concentration was well advanced as early 

 as Cretaceous . . . and the process of enrichment has undoubt- 

 edly continued until the present time. It therefore appears that 

 the circulating waters have had eras in which to perform their 

 work ; indeed a part of the pre-Paleozoic time and all of the 

 Paleozoic, Mesozoic, and Cenozoic." A like analysis will dis- 

 pose of the argument relative to the presence of magnesia in 

 the older rocks. Dolomitic rocks are now, I believe, univer- 

 sally recognized in the majority of cases as due not to original 

 sedimentation, but to a gradual replacement of lime by mag- 

 nesia through the downward percolation of very dilute magne- 

 sian waters.^ There is no reliable evidence showing that early 

 sea waters were richer in magnesian salts than those of to-day ; 

 that more magnesia is to be found in the older limestones 

 is due wholly to prolonged periods of a process of concentration 

 still in operation. 



* Monograph 52. U. S. G. S., The Geology of the Lake Superior Eegion. 

 f Loc. cit., p. 558. 



j:See Rep. of Coral Eeef Com. Royal Soc. of London, 1903; also Clarke's 

 Summary, Bull. 491, U. S. G. S., pp. 534-45. 



