858 



SCIENCE. 



[N, S. Vol. XXIV. No. 62 ( 



matters, but the following touches one of 

 much importance. The reviewer says that the 

 reasons are not clear for the inference that ' a 

 vegetal covering of the land extended as far 

 back in the history of the earth as clay slates, 

 quartzose sandstones and limestones formed 

 the prevailing sediments.' He proceeds to 

 give reasons for thinking that decomposition 

 and the deposition of limestone may take place 

 without the aid of vegetation, overlooking the 

 meaning of the phrase 'the prevailing sedi- 

 ments,' and that of the context, in which it is 

 stated that " if the surface be bare of vegeta- 

 tion, the crystalline rocks are usually disag- 

 gregated before they are decomposed, since 

 destructive action occurs best at the junctions 

 of crystals and along cleavage lines and hence 

 the crystals are usually separated from one 

 another before they are fully decomposed. In 

 the absence of a covering to hold them in 

 place until they are decomposed they are apt 

 to be washed away, and the resulting deposit 

 consists in considerable part of grains of 

 feldspar, mica, hornblende and other minerals 

 which do not usually occur in well decomposed 

 sediments." It is difficult to see how a care- 

 ful student can fail to note that there is here 

 a recognition of decomposition, on a small 

 scale, independent of vegetation. The point 

 to be emphasized was, on the other hand, that 

 in the absence of the protection of a vegetal 

 covering mechanical disintegration so far over- 

 powers decomposition that in most cases the 

 disaggregated particles on the naked surface 

 are carried away by erosion, and give rise to 

 a formation which is only partially decom- 

 posed. When, therefore, the 'prevailing 

 formations' — not exceptional or possible ones 

 —consist of the products of mature decom- 

 position (II., p. 199, where the matter is again 

 stated) it is reasonable to suppose that the 

 land possessed a vegetal covering. 



The authors are scored for not treating the 

 Pacific coast geology of Mesozoic and Ter- 

 tiary times more fully. It may be that they 

 would reply that it is because relatively few 

 good sections of the strata of the Pacific coast 

 have been published. Nevertheless, sections 

 from that part of the country, illustrating the 

 systems referred to appear as figures on no 



less than twelve pages in volume III. and 

 additional columns are furnished at the close 

 of that volume. These sections receive as 

 much consideration in an interpretative way 

 as one could expect in a treatise so cosmo- 

 politan in its field. Not improbably the geolo- 

 gists of Dakota think that the geology of 

 Dakota has not received adequate recognition, 

 while the geologists of Texas have the same 

 feeling with reference to the geology of their 

 state, and so on indefinitely. It is but natural 

 that those regions which have been most thor- 

 oughly investigated should receive the. largest 

 share of attention. 



It is in view of such points as these that 

 the reviewer takes occasion to say that ' these 

 are oversights which must annoy teachers of 

 geology.' It is the present writer's observa- 

 tion, in using these volumes as a text-book 

 with students, that the number of misinter- 

 pretations put upon the text is extremely 

 small; and while teachers of geology may re- 

 gret any and every imperfection in the vol- 

 umes they may well be gratified that so com- 

 plete and readable a treatise is now available. 

 Eliot Blaokwelder. 



University of Wisconsin, 

 November 7, 1906. 



THE DETERMINATION OF THE TYPES OF GENERA. 

 A CORRECTION. 



In my recent article on the ' Determination 

 of the Types of Genera '^ the sentence," " As 

 gryphus was the last species removed from the 

 genus Vicltur it is its type by elimination 

 ■X- * -Kyj requires modification. In reality 

 gryphus was the second species removed from 

 the genus, but the last species to be disposed 

 of by assignm.ent to a special genits of its 

 own, which is the statement intended. From 

 the modern standpoint, no two of the original 

 six species of Vultur were congeneric, and 

 gryphus was the last species to be provided 

 with a distinctive generic name. 



J. A. Allen. 



^ " The ' Elimination ' and ' First Species ^ 

 Methods of Fixing the Types of Genera," Science,^ 

 N. S., Vol. XXIV., No. 624, pp. 773-779, No- 

 vember 14, 1906. 



^ Page 776, at middle of first colmim. 



