996 



SCIENCE. 



[N. S. Vol. V. No. 130. 



to find fault with in regarding the book from 

 this standpoint. 



The chapter on Igneous Kocks is a handy 

 compendium of recognized types. The classi- 

 fication makes mode of occurrence and age of 

 secondary importance, and in this respect dif' 

 fers from while it gains in clearness upon the 

 classification of Eosenbusch. While this por- 

 tion of the book is rich in reference to rock 

 types, it will hardly serve (as the author him- 

 self indicates by reference to special treatises) 

 as a means of identifying igneous rocks. 



The chemically formed rocks are described 

 along with the clastic rocks under the heading 

 of Aqueous Eocka. From the point of view of 

 the genesis of these two groups of rocks it seems 

 objectionable to classify them in this manner. 

 Many of the chemically formed rocks are 

 closely allied to the igneous rocks. Moreover, 

 in geology, the term ' aqueous rocks ' has been 

 from the days of Lyell intimately associated 

 with the fragmental rocks alone. The same 

 extension of use for this term would embrace 

 igneous rocks, in which the action of water is 

 largely concerned. 



It is to be noted that Griswold is misquoted 

 in regard to the origin of the novaculites of 

 Arkansas. Instead of supposing these rocks to 

 be ' a chemical deposit in the form of a siliceous 

 slime on a sea-bottom ' (p. Ill), this geologist, 

 according to the very report quoted by Profes- 

 sor Merrill, makes the novaculites a deposit of 

 very fine fragmental silica, almost without 

 other materials laid down far from the shore- 

 line in the form of mud or ooze. In the case of 

 other quotations it is sometimes doubtful as to 

 which author the work in a certain district is to 

 be referred. One must look up these refer- 

 ences, as, of course, the exhaustive student 

 will take care to do. 



The author introduces a few terms not before 

 used in text-books. Colluvial, a name pro- 

 posed by Professor Hilgard and here restricted 

 to talus and cliflf debris and avalanche material, 

 appears to be inappropriate for deposits which 

 owe their transportation to gravity rather than 

 to running water. The time has undoubtedly 

 come when this class of detritus should be given 

 a distinctive name implying the mode of depo- 

 sition, but the idea of water action, so clearly 



expressed in colluvial, is contradictory to the 

 essential fact which distinguishes these deposits 

 from alluvium. Regolith is a term proposed by 

 the author for the superficial deposits or uncon- 

 solidated materials, the products of disintegra- 

 tion and decomposition. It is difiicult to realize 

 that this term has advantages over those it is 

 designed to displace, since the superficial de- 

 posits of the earth form a very imperfect blanket, 

 pierced by every outcrop of bed-rock and moun- 

 tain peak, worn through along every rocky river 

 bed and sea-cliff". We might take a hint from 

 the quarryman's 'topping' and our ' eplgene 

 processes ' to speak of the epilith, if we must 

 have a Greek word for the ' waste ' of the land. 



Professor Merrill clearly points out the diflfer- 

 ence between metamorphism and weathering, 

 and between atmospheric decay proper and 

 those deep-seated changes which are sometimes 

 included under this head. 



Special importance is attached to the distinc- 

 tion between decomposition and disintegration 

 in the case of the feldspars. Orthoclase may 

 pass into the state of fine silt without actual de- 

 composition. Such breaking up must be dis- 

 tinguished from decomposition, as it takes place 

 in the lime-soda feldspars. Fournet is quoted 

 as stating that hornblende yields less readily to 

 decomposition than feldspar, while Becker holds 

 the opposite as true. It may be noted here 

 that in the Carboniferous arkoses of New Eng- 

 land, derived from disintegrated hornblendic 

 granitites, hornblende is invariably absent from 

 the bleached basal sediments, the feldspar frag- 

 ments having been evidently strong enough af- 

 ter the breaking down of the granitite to with- 

 stand transportation along with grains of quartz. 

 It can hardly be said that the heavier but 

 smaller hornblende grains would be altogether 

 assorted out from the quartz and feldspar by 

 mechanical means. The hornblende must have 

 decomposed before the transportation of the 

 particles. 



The concretionary forms of joint-blocks in 

 some igneous rocks and the huge granite bosses 

 are thought by Professor Merrill to be alike an 

 effect of weathering. Even the spheroidal 

 structure of basalt, though admitted in defer- 

 ence to the opinion of some writers to be an 

 original structure, is apparently in the author's 



