March 9, 1S83.] 



SCIENCE. 



141 



ronian. The Canadian geologists liave fallen into the 

 custom of calling every thing Huronian that is schis- 

 tose, and yet it is evident tliat much of the schists 

 called by them Huronian are but dependencies of the 

 older gneiss. I may say in tliis connection, that the 

 'Aniniilvie group' of Thunder Bay, wliich Selwyn, 

 following Logan, refers to the cojiper-bearing series, 

 is, beyond question, the exact equivalent of the un- 

 folded iron-bearing rocks of the Penokee region of 

 Wisconsin, and these again of tlie folded iron-bearing 

 schists of the Marquette and Menominee regions ; and 

 that there can be little doubt that all of these ai'e the 

 equivalents of the original Huronian of the north 

 sliore of Lake Huron. This reference of the Anirai- 

 kie rocks to the Huronian is, I know, a novel position, 

 although Logan long since for a time held the same 

 view; but I feel confident tliat it is a correct one. 

 Indeed, I speak confidently as to all of the conclusions 

 here mentioned, because I have had unusual oppor- 

 tunities for observation, liaving studied both the 

 Cambrian sandstones and the copper-bearing i-ocks, 

 as well as the Huronian from Keweenaw Point across 

 Wisconsin, into Minnesota, and thence north-east- 

 ward to Thunder, Black, and Nipigon Bays. Having 

 made this wide sweep, I can see quite well how others, 

 examining only portions of the district, should be 

 puzzled or reach different conclusions. 



There is one other statement in Mr. Selwyn's letter 

 that I cannot concur in; and that is as to the occur- 

 rence of tuffs, or volcanic detrilal matter, among the 

 copper-bearing rocks. I know such materials should 

 be expected to occur in a series largely composed of 

 volcanic flows; but after a careful search for them in 

 the field, and the study of a large number of thin 

 sections, I can find no fragmental rocks which are 

 not either certainly ordinary sediments or at least 

 much more probably so than of direct volcanic origin. 



Madison, Wis., Feb. 16, 1883. K. D. IRVING. 



WHITNEY'S CLIMATIC CHANGES. 



The climatic changes of later geological times : a 

 discussion based on observations made in the Cor- 

 dilleras of North America. By J. D. Whitney. 

 Cambridge, 1882. 14-f 394 p. 4°. 



I. 



This volume is one of a series, by the same 

 author, based ou the worlv of the California 

 geological survej-, but published under tlie aus- 

 pices of the Museum of comparative zoSlogy. 

 The preceding vohime treated of the auriferous 

 gravels of California, and this one is in some 

 sense a sequel to it. Although the treatise is 

 an outgrowth of the Califoi-nian work, its ma- 

 terial includes observations by the author in 

 eastern America and in Europe, as well as 

 data gathered by others from all regions. It 

 is of interest, not only by reason of its contri- 

 bution of original matter, but because it devel- 

 ops at length a theory that has heretofore been 

 stated but brieflj-, and which has been almost 

 ignored b^' the advocates of its rivals. The 

 book comprises four hundred quarto pages, but 

 is without index, — an omission onlj^ imper- 

 fectly supplied bj- an analytic table of contents. 



In the volume ou the Auriferous gravels, our 



author states that the Sierra Nevada has had 

 substantially the same height and dimensions 

 from cretaceous time. The streams which 

 flowed down its western flank during the ter- 

 tiaiy did uot excavate gorges, but, on the 

 coutraiy, spread gi'eat bodies of detritus. The 

 modem rivers, following essentially the same 

 courses, have cut deep V-shaped caiions, which 

 were partiall3- filled with ice during the glacial 

 epoch. The tertiaiy climate was relativel.y 

 moist, as is shown hy the broad channels of 

 the tertiaiy rivers, and bj' the fact that they 

 filled their valley's with gravel instead of cut- 

 ting caiions. 



In the present volume, the idea of a diminu- 

 tion of precipitation from pliocene to present 

 time is expanded into a theoiy of general, 

 continuous, secular desiccation, and is devel- 

 oped at length. Evidence is adduced to show, 

 that within historic time there has been a 

 shrinking of lakes and rivers in South America, 

 in the interior basin of Asia, and about the 

 shores of the Mediterranean ; and that, in late 

 geological time, large areas in northeastern 

 and northwestern Asia and northern Africa 

 were covered with water, while the Great Basin 

 of North America contained a system of fresh- 

 water lakes. The ancient glaciers of the Sierra 

 Nevada, and of the Cordilleras generally, are 

 described ; and their disappearance is referred 

 to the same desiccation. An account is given 

 of the tertiaiy lakes of western North America, 

 and it is pointed out that their extent gradually 

 diminished. The popular theory that modern 

 desiccation is due to the destruction of forests, 

 and the theory of some geologists that the 

 great lakes and rivers of the immediate past 

 wei'e connected with the melting of the ice of 

 the glacial epoch, are controverted ; and it is 

 argued that all the phenomena pertain to a 

 general, secular diminution of precipitation. 



To account for tliis diminution, the following- 

 considerations are adduced : The amount of ■ 

 moisture precipitated to the earth depends on 

 evaporation. The amount of evaporation de- 

 pends on temperature and on the extent of 

 water-surface. If, therefore, it can be shown 

 that the continents of the earth have gradually 

 increased in area, while the oceans have gradu- 

 ally diminished, or if it can be shown that the 

 temperature of the atmosphere has gradually 

 lowered, then an explanation will be afforded 

 of the change in precipitation. After a review 

 of the facts. Professor AVhitney concludes that 

 an expansion of continents has actuallj- taken 

 place, but that it is inadequate to account for 

 the observed recent desiccation. He therefore 

 bases his theoiy chiefly upon a loss of heat. 



