NATURE 



97 



THURSDAY, JUNE 3, 1897. 



ROCK- WE A THERING. 

 A Treatise on Rocks, Rock-weathermg and Soils. By 



George P. Merrill, Curator of Geology in the U.S. 



National Museum. Pp. xx + 411. (New York : The 



Macmillan Co. London : Macmillan and Co., Ltd., 



1897.) 

 " '' I ""HE ruins of an older world are visible in the present 

 J- structure of our planet ; and the strata which now 

 compose our continents have been once beneath the sea, 

 and were formed out of the waste of pre-existing conti- 

 nents. The same forces are still destroying, by chemical 

 decomposition or mechanical violence, even the hardest 

 rocks, and transporting the materials to the sea, where 

 they are spread out, and form strata analogous to those 

 of more ancient date." These memorable words of 

 Hutton, written more than a century ago, have been 

 appropriately selected by the author as the motto for 

 his work, the main object of which is to describe the 

 various processes of rock degeneration and soil forma- 

 tion. 



The book is divided into five parts, dealing with the 

 constituents of rocks, the different kinds of rocks, the 

 weathering of rocks, the transportation and re-deposition 

 of rock debris, and the mantle of unconsolidated material 

 which covers the greater portion of the land surface, and 

 for which the author proposes the term " regolith." 



The first two parts are not remarkable, either on 

 account of the matter contained or the mode of treat- 

 ment. The author somewhat disarms the critic, so far 

 as these parts are concerned, by stating in the preface 

 that " the work must be considered in no sense a 

 petrology, as this word is commonly used " ; neverthe- 

 less, we feel justified in pointing out that much which is 

 inserted might have been omitted without detriment to 

 the work so far as its main object is concerned, and 

 much that is omitted might appropriately have found a 

 place. Thus the first part contains scraps of inform- 

 ation about the characters of rock-forming minerals, 

 which are insufficient for the beginner, and are not re- 

 quired by the advanced student. More serious are the 

 omissions. In a work dealing with the disintegration 

 and decomposition of rocks, we naturally look for a 

 somewhat detailed account of the pathological characters 

 of the primary constituents of the crystalline rocks, and 

 of the physical and chemical conditions under which the 

 various secondary products arise. Very little information 

 is, however, given on these points ; and this is all the 

 more remarkable, because the works of Roth, and the 

 numerous and important papers by Lemberg, referred to 

 by the author, would have readily supplied him with a 

 large amount of the necessary material. 



The classification of rocks adopted by the author is 

 simple, natural, and well suited to his purpose. The 

 igneous rocks are divided into nine groups, depending 

 mainly on chemical composition, and are defined by 

 taking the plutonic and volcanic representatives of the 

 several groups. Thus we have the granite-liparite group, 

 the foyaite-phonolite group, the gabbro-basalt group, 

 and so on. The author does not favour the modern 

 NO. 1440, VOL. 56] 



tendency to introduce new names for comparatively un- 

 important local varieties, and protests against "such 

 monstrosities of nomenclature as ovachichite, monchi- 

 quite, yogoite, and absarokite." 



The descriptions of the rocks are generally accurate, 

 but we have noted one or two imperfections. Thus the 

 statement that " chert is an impure flint, containing not 

 unfrequently nummulitic remains," appears somewhat 

 strange when one remembers that cherts are of all ages, 

 and that they are largely due to accumulations of sponge 

 spicules and radiolaria. 



The unpleasant task of fault-finding ends with the 

 consideration of Part H. In the remaining parts which 

 make up the main body of the work we have an 

 admirable presentation of the various phenomena con- 

 nected with the disintegration and decomposition of 

 rocks, the transportation of rock-debris, and the for- 

 mation of soil. Part III. deals with weathering. The 

 action of the atmosphere, the chemical action of water, 

 the mechanical action of water and ice, and the action 

 of plants and animals, including the nitrifying organism, 

 are severally discussed in detail. Then follows a special 

 chapter, based largely on the author's original work, and 

 containing some hitherto unpublished material, in which 

 special cases of decomposition are described. The 

 cases have been carefully selected, chemical analyses of 

 the fresh and decomposed rocks are given, and these are 

 supplemented .by mechanical analyses of the latter. 

 In a few instances, chemical analyses of the different 

 parts into which the decomposed rock can be separated 

 by mechanical means are also given. This exhaustive 

 study necessarily brings out a number of points of great 

 interest, only one or two of which can be referred to. 



To illustrate the method of treatment, we quote below 

 a table representing the analyses of fresh and decom- 

 posed gneiss from Albemarle County, Virginia — that is, 

 from a region outside the area of glaciation, and one in 

 which the rocks have, therefore, been subjected for a 

 long period to atmospheric influences. 



A comparison of the bulk analyses of the fresh rock, 

 and the incoherent material resulting from its decom- 

 position, necessarily gives, in many cases, a very imperfect 



1 Gain. 



F 



