October 19, 1893] 



NATURE 







89 



best. " I have written," says the author, " not for 

 wealthy amateurs, nor for people who do not care to 

 think, but for men and women who have to give up 

 something else to spend a sovereign on their own educa- 

 tion. Nearly all the apparatus described in this book 

 can be made by anyone with a few tools and a little 

 finger-skill." In conformity with this laudable desire, 

 technical terms are rarely introduced without being ex- 

 plained, and by simple words and apt illustration the 

 way to electrical knowledge is made as easy and pleasant 

 as it possibly could be. Indeed, popularity of style ap- 

 pears to be the book's sole raison cVctri:, for, with one or 

 two exceptions, the facts described are to be found in a 

 number of elementary text-books. However, it can be 

 said that there are very few, if any, books of the modest 

 dimensions of the one before us in which so much in- 

 formation is imparted in a more popular manner. The 

 descriptions of experiments and principles are easy read- 

 ing without being diffuse ; the hydrostatic and other 

 analogues are numerous, yet they are never used where 

 likely to lead to a misconception. The illustrations, 

 however, are not worthy of the text. They should 

 have been far more numerous and less sketchy in order 

 to appeal to the public for whom the book has been 

 specially designed. 



LETTERS TO THE EDITOR. 



[ The Editor does not hold himsetf responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the 'writers of, rejected 

 manuscripts intended for this or any other part o/' Nature. 

 No notice is taken of anonymous communications.'^ 



The Supposed Glaciation of Brazil. 



In the second volume of Nature, p. 510, I reviewed the 

 late Prof. Hartt's "Geology and PhysicalGecgraphy of Brazil," 

 and called attention to the author's views, as well as those of 

 the late Prof. Agassiz, relating to the supposed glaciation of that 

 country. From their very positive statements I concluded that 

 the evidence as described by them did actually exist, and that 

 until it was disproved it should not be ignored. In my 

 " Darwinism," p. 370, I stated, on the authority of my fiiend, 

 Mr. J. C. Branner, now Professor of Geology in the .Stanford 

 University, California, who succeeded Prof Hartt in Brazil, 

 and had a much more extensive knowledge of the country, that 

 the supposed glacial drift and erratic blocks were all results of 

 subaerial denudation. Recently, however, Sir Henry Howorth 

 has quoted some passages from my review in illustration of the 

 wild and incredible theories of some geologists, as samples, in 

 fact, of the " Glacial Nightmare " ; and, as no authoritative dis- 

 proof has yet been given of the exceedingly strong and positive 

 statements of Agassiz and Hartt, I beg leave to lay before the 

 readers of Nature some extracts from a paper on " The Sup- 

 posed Glaciation of Brazil," by Prof. Branner, which will shortly 

 be published, and of which he has kindly sent me a type-written 

 copy in advance. As a partial justification of what has now 

 proved my too hasty acceptance of the statements of these gentle- 

 men, I will give one passage in which Prof. Agassiz refers to 

 the supposed glacial phenomena near Ceara : — "I may say that 

 in the whole valley of Hash there are no accumulations of 

 morainic materials more characteristic than those I have found 

 here, not even about the Kirchel ; neither are there any remains 

 of the kind more striking about the valleys of Mount Desert in 

 Maine, where the glacial phenomena are so remarkable ; nor in 

 the valleys of Loch Fine, Loch Awe, and Loch Long, in Scot- 

 land, where the traces of ancient glaciers are so distinct." Both 

 Agassiz and Hartt were equally strong as to similar phenomena 

 near Rio. 



It is to be first noted that Hartt had only spent eighteen 

 months in Brazil when he wrote his book, and his views on the 

 glacial phenomena were thus based on a very hasty survey of 

 that enormous territory. Piof. Branner went with him when 

 he again visited Brazil in 1874, helped him in his geological 

 work till his death in 1877, and himself remained five years 

 longer making a geological survey of the country ; and he states 

 that, before his death, Hartt's views underwent a radical change. 

 Prof. Branner says : — 



NO. T25I, VOL. 48] 



" Under his direction I did more or less work in the moun- 

 tains about Rio de Janeiro for the purpose of sifting the evidence 

 of glaciation in that region, and I am glad to say, in justice to 

 the memory and scientific spirit of my former chief and friend, 

 that long before his death he had entirely abandoned the theory 

 of the glaciation of Brazil, and that the subject had ceased to 

 receive further attention, even as a working hypothesis." 



A few extracts must now be given showing to what causes the 

 phenomena which deceived these observers are really due. And 

 first as to what were supposed to be erratic boulders often em- 

 bedded in boulder clay. 



"The boulders believed to be erratics are not erratics in the 

 sense implied, though they are not always in place. The first 

 and most common are boulders of decomposition, either 

 rounded or subangular, left by the decay of granite or gneiss. 

 Sometimes they are embedded in residuary, and therefore un- 

 stratified, clays, formed by the decomposition in place of the 

 surrounding rock. And everyone has heard of the great depth 

 to which rocks are decomposed in Brazil. The true origin of 

 these boulders and their accompanying clays is often obscured 

 by the ' creep ' of the materials, or in hilly districts by land 

 slides, great or small, that throw the whole mass into a confusion 

 closely resembling that so common in the true glacier boulder 

 clays. In this connection too much stress cannot be placed upon 

 the matter of land slides ; they are very common in the hilly 

 portions of Brazil, and aside from profound striations and facet- 

 ting produce phenomena that, on a small scale, resemble glacial 

 till in a very striking degree." .... 



"The second method by which these boulders have been 

 formed is quite similar to the first, but instead of being 

 cores of granite or gneiss, they have been derived by the same 

 process of exfoliation and decomposition from the angular 

 blocks into which the dikes of diorite, diabase, or other dark- 

 coloured rocks break up. Their colour marks them as quite 

 different from the surrounding granites, and the dikes themselves 

 are almost invariably concealed. The residuary clays derived 

 from the decomposition of these dikes are somewhat different 

 in colour from those yielded by the granites, so that when 

 'creep' or land-slides add their confusion to the original 

 relations of the rocks the resemblance to true glacial boulder 

 clays is pretty strong. The chance of discovering the source 

 of such boulders is further decreased by the depth^to which the 

 mass of the rock has decayed, and by the inpenetrable jungles 

 that cover the whole country, and so effectually limit the range 

 of one's observations. Dikes, such as these last mentioned, 

 are not uncommon in the mountains about Rio de Janeiro. 

 Indeed, what have generally been regarded as the very best 

 evidences of Brazilian glaciation, some of the boulders near the 

 English hotel at Tijca, fall under this head, though some are 

 of gneiss. The fact is that the great mountain masses about Rio 

 are of granite or gneiss, while some of the boulders come from 

 the dikes of diabase or other dark-coloured rock high on their 

 sides — dikes which were not visited by Agassiz or Hartt." 



Prof. Branner then describes a third class of supposed erratic, 

 derived from certain sandstone beds of the tertiary deposits, 

 which, by exposure, change to the hardest kind of quartzite, 

 and when the surrounding strata are j-emoved by denudation, 

 and a few blocks of this quartzite are left, they are so unlike the 

 rocks by which they are surrounded that unless the observer has 

 given a special study to the tertiary sediments, he is liable to 

 be misled by them. 



The wide-spread coating of drift-like materials tha covers 

 considerable areas of the country, consisting of boulders, 

 cobbles, and gravels, sometimes assorted and sometimes having 

 clay and sand mixed with them, are then described, and are 

 shown to be due to the denudation of the tertiary beds during 

 the last emergence of the land, aided by subsequent subaerial 

 denudation and surface wash. Prof. Branner thus concludes : — 



"I may sum up my own views with the statement that I did 

 not see, during eight years of travel and geological obser- 

 vations that extended from the Amazon valley and the coast 

 through the highlands of Brazil and to the head waters of the 

 Paraguay and the Tapagos, a single phenomenon in the way of 

 boulders, gravels, clays, soils, surfaces, or topography, that 

 required to be referred to glaciation. " 



The very clear statement above given of the real nature of the 

 phenomena which deceived Prof Agas.sizand Mr. Hartt, is very 

 instructive, and it shows us that a superficial resemblance to 

 drift, boulder-clay, and erratic blocks, in a comparatively un- 

 known country, must not be held to be proofs of glaciation. 



