Mat 30, 1919] 



SCIENCE 



519 



is the account of the Bolivian tin veins. Pro- 

 fessors Miller and Singewald were fortunate 

 in finding fossil plants in the shales at Potosi, 

 and as a result of this discovery were able to 

 establish that the tin veins at Potosi were 

 formed in Pliocene or Pleistocene time. This 

 is a remarkable conclusion and shows that 

 these wonderfully productive tin lodes are in 

 a geological sense extremely youthful; in fact 

 they are probably the most youthful econom- 

 ically valuable mineral deposits of first rank 

 in the world. Professors Miller and Singe- 

 wald extend this age determination to all the 

 Bolivian tin veins and maintain that they are 

 all of Pliocene age. This conclusion may or 

 may not be true, for the veins of the different 

 districts appear to be associated with igneous 

 rocks of a wide range of texture: pegmatites, 

 aplites, granite, granite porphyry, rhyolite 

 porphyry, rhyolite and " true quartz porphyry." 

 As a matter of fact, no thorough field study 

 of the Bolivian tin veins as a whole has yet 

 been made. The studies hitherto made have 

 been mainly petrographic, by geologists who 

 have not collected the specimens they studied. 

 It is not to be expected that a very deep in- 

 sight into the fundamental problems could be 

 attained by that method. Even in such a 

 relatively subordinate matter as the nomen- 

 clature of the igneous rocks the petrologist 

 has felt it necessary to use such obsolescent, 

 non-committal terms as quartz porphyry to 

 describe some of the rocks to which certain 

 Bolivian tin veins are genetically related. 

 When field work becomes the main method of 

 attack and the microscope is used as an 

 auxiliary — a powerful auxiliary it is trud^ 

 more satisfactory results will be attained, and 

 it is therefore a pleasure .to learn that Pro- 

 fessor Singewald is returning to Bolivia in 

 order to take up a careful study of the tin 

 veins in their broader geologic aspects. 



Another district of special interest is Coro- 

 coro in Bolivia, which like the Lake Superior 

 district is one of the world's two productive 

 copper districts in which the chief ore mineral 

 is native copper. Brazil holds the distinction 

 of having in the Morro Velho mine the deep- 

 est mine in the world, the lowest workings 



having attained a vertical depth of 6,128 

 feet. The ore on the lower levels averages 

 nearly $13 a ton in gold and indicates an ex- 

 traordinarily long vertical range of gold-ore 

 deposition. Apparently not much is known 

 about the geology of this remarkable ore body, 

 however. There are many other interesting 

 deposits described in the book, but it would 

 lengthen this review unduly even briefly to 

 call individual attention to them. The out- 

 standing feature of the economic geology of 

 the South American continent is its preem- 

 inence in the number of its geologically 

 youthful primary ore deposits of the first 

 order of magnitude. 



Professors Miller and Singewald have placed 

 all interested in the mineral resources of 

 South America imder a deep debt for the 

 labor they have expended in marshalling the 

 widely scattered information and for present- 

 ing it attractively in a condensed and easily 

 usable form. They can be gratefully assured 

 that they have filled a genuine want in the 

 literature of economic geology. 



Adolph Knopf 



United States Geological Survey 



THE ECOLOGY OF NORTH AMERICAN 

 LYMN.ffiID.ffi: 



In a recent paper in Science' the following 

 statement appears : " There are three groups 

 of limnieas found in North America, the 

 abysmal limnseas including Lymnwa (Acella) 

 haldemani Binney, the moss limnseas includ- 

 ing Lymnwa (Galha) truncatula Miill., hu- 

 milis Say, and the marsh limnaias including 

 Lymnwa siagnalis, L. columella," etc. This 

 classification of our pond snails is so unusual 

 and so far from representing the true ecolog- 

 ical relations of this group, as well as of the 

 allied groujw Planorhis and Physa, that a few 

 ob.?ervat)ons on the subject seem necessary. 



As far as known there are no abysmal 

 lymna?as or other fresh-water pulmonatcs in 

 America, comparable to the true abysmal 

 fauna of the deep lakes in Switzerland, where 

 Lymnwa stagnalis occurs in Lake Geneva at 

 a depth of 250 meters and Lymnwa ahyssicola 



1 Science, N. S., XLVIII., p. 578, 1918 . 



