IOWA ACADEMY OF SCIENCES. 101 



The explanation of ore-deposits lies neither wholly 

 within an aqueous theory, nor wholly within an igneous 

 theory. It lies rather between the two, or combines both. 



The most recent rock analyses, in which the rarer ele- 

 ments have been especially tested for, show appreciable 

 amounts of substances which ordinarily are calculated in 

 with more abundant compounds. In these new experi- 

 ments the microscope has been of inestimable value. 

 These more modern refinements in rock analysis demon- 

 strate beyond all doubt that many of the less common 

 metals and rare earths exist in some form or other in 

 nearly all crystalline rocks, and in most of them in much 

 greater abundance than has been generally supposed. 



In the old pre-Cambrian igneous rocks of Missouri, which 

 have not been affected by orogenic movements to any 

 appreciable extent, and where surrounding conditions 

 appear to preclude the secondary introduction of metallic 

 salts, Robertson found measurable quantities of lead, zinc, 

 copper and manganese. It is a noteworthy fact that the 

 percentages of the metals increased rapidly with the 

 increase in the amount of ferro-magnesian silicates present, 

 in the rocks. The diabases contained five times as much 

 metal as the granites poor in dark mica. Another feature 

 to be noted is the fact that while lead and zinc occur in 

 about equal proportions, amounting to, in some cases as in 

 the diabase of Shrainka, one-half of oue per cent of the 

 constituents soluble in nitro-hydro-chloric acid, the last 

 mentioned is almost wholly absent among the ore deposits 

 of the eastern slope of the Ozark dome, where the first 

 mentioned metal is the chief ore. 



Most of the attempts to determine the metallic element 

 of the rocks have been through chemical analysis. For 

 the purpose of reaching definite conclusions, the results as 

 a whole have been far from being satisfactory. So far as 

 the chemical methods and manipulation go, results have 

 been exact enough, but there are sources of error which 

 have not been guarded against which lie beyond the sphere 

 of the chemist. 



