October 28, 1892.] 



SCIENCE. 



245 



sembling the real one ; this, apart from errors or transposition of 

 labels, to which accidents all collections are more or less liable, in 

 proportion to their Hge. 



While, therefore, fully admitting the great value of a type, or 

 type-figure, it is necessary to ascertain that it is really the specimen 

 or represents the specimen originally described. If it contradicts 

 the original description in any important respect, and especially 

 if it is an insect known to be from a different locality to that 

 assigned to it by the original describer, it is more than probable 

 that it is not the original type at all, and is worse than mislead- 

 ing. Errors of locality are always possible ; but much will depend 

 on the author. Donovan, for instance, was extremely careless 

 about localities, but, as he figured all his species, this matters 

 less; on the other hand, Fabricius was far more careful than later 

 authorities have given him credit for; and an error of this kind in 

 his work was quite exceptional. 



THE CONVEX PROFILE OF BAD-LAND DIVIDES. 



BY W. M. DAVIS, HARVABD COLLEGE, CAMBRIDGE, MASS. 



In Mr. Gilbert's analysis of land sculpture, constituting chapter 

 V. of his "Geology of the Henry Mountains," he explains why the 

 surface of an eroded region possesses slopes that are concave up- 

 wards and steepest near the divides, and shows that it is for the 

 reasons there stated that mountains — that is, mature and well- 

 sculptured mountains, such as are of ordinary occurrence — are 

 steepest at their crests (p. 116). The aretes of the Alps illustrate 

 this perfectly. Gilbert calls this generalization the "law of di- 

 vides." 



But in discussing the forms assumed by eroded bad-lands, or 

 arid regions of weak structure with insignificant variety of text- 

 ore, he finds an exception to the law of divides. The two lateral 

 •concave slopes of a bad-laud ridge do not unite upwards at an 

 angle, forming a sharp divide, but are joined in a curve that is 

 convex instead of concave upnards. "Thus in the sculpture of 

 the bad lands there is revealed an exception to the law of divides, — 

 an exception which cannot be referred to accidents of structure, 

 -and which is as persistent in its recurrence as are the features 

 which conform to the law, — an exception which in some unex- 

 plained way is part of the law. Our analysis of the agencies and 

 conditions of erosion, on the one hand, has led to the conclusion 

 that (where structure does not prevent) the declivities of a con- 

 tinuous drainage-slope increase as the quantities of water flowing 

 over them decrease ; and that they are great in proportion as they 

 -are near divides. Our observation, on the other hand, shows that 

 the declivities increase as the quantities of water diminish, up to 

 a certain point where the quantity is very small, and then de- 

 <;rease : and that declivities are great in proportion as they are near 

 divides, unless they are very near divides. Evidently some factor 

 has been overlooked in the analysis, — a factor which in the main 

 is less important than the flow of water, but which asserts its ex- 

 istence at those points where the flow of water is exceedingly 

 small, and is there supreme" (pp. 1^3, 133). 



It has for some time seemed to me that the overlooked factor 

 is the creeping of the surface soil ; and, as I have not seen men- 

 tion of this process as bearing on the form of the crest-lines of 

 divides, a brief note on the subject is here offered. 



The superficial parts of rock-masses are slowly reduced to rock- 

 waste or soil by the various processes included under the term, 

 weathering. Unconsolidated materials are in the same way re- 

 duced to finer texture near their surface. The loose and often 

 fine material thus provided at the surface is carried away by 

 various processes, of which the chief are moving water, moving 

 air, and occasionally moving ice ; but there is an additional process 

 of importance, involving dilatation and contraction of the soil, and 

 in consequence of which not only the loose particles on the surface 

 are transported, but a considerable thickness of loose material is 

 caused to creep slowly down- hill. 



Dilatation is caused by increase of temperature, by increase of 

 moisture, and by freezing. Vegetable growth may probably be 

 a,dded to this list. The movements are minute and slow. They 

 are directed outwards, about normally to the surface. Contrac- 

 tion follows dilatation, when the soil cools or dries, or when its 



frost melts. The movement of the parts is then not inward at a 

 normal to the surface, but vertically downwards, or even down- 

 wards along the slope. As the two motions do not counterbalance 

 each other, a slow down-hill resultant remains. This is greatest 

 near the surface, where the dilatations and contractions are great- 

 est; but it does not cease even at a depth of several feet, perhaps 

 of many feet. Hence the down-hill dragging of old-weathered 

 rock, often well shown in fresh railroad cuttings in non-glaciated 

 regions. I presume all this is familiar to most readers; although 

 from the frequent inquiry concerning the means by which valleys 

 are widened it is evident that the creeping process is not so gen- 

 erally borne in mind as that by which running water washes loose 

 material down-hill. 



The form assumed by the surface of the land depends largely 

 on the ratio between the processes of washing and creeping. 

 Wherever the concentration of drainage makes transportation by 

 streams effective, the loose material is so generally carried away 

 (except on flood-plains) that the action of creeping is relatively 

 insignificant But on divides, where drainage is not concentrated 

 but dispersed, the ratio of creeping to washing is large, even 

 though the value of creeping is still small. This is especially the 

 case in regions of loose texture and of moderate rainfall; that is, 

 in typical bad-lands, where the supply of loose surface- material 

 ready to creep is large, and where the loose material is slowly 

 taken away by washing. On the divides of such regions, the 

 surface form is controlled by the creeping process. The sharp- 

 edged divides, that should certainly appear if washing alone were 

 in action, are nicely rounded off by the dilatations and contrac- 

 tions of the soil along the ridge-line. The result thus determined 

 by the slow outward and downward movements of the particles 

 might be imitated in a short time by a succession of light earth- 

 quake shocks. 



Mr. Gilbert has himself given several beautiful illustrations of 

 the close dependence of sharp or rounded divides on rainfaU; 

 structure remaining constant. If the rainfall should increase in 

 bad-land regions, would not all their divides become sharper; 

 and if the rainfall were continuous, so as to carry away every loose 

 particle as soon as it is loosened, would not the divides assume 

 the sharp ridge-line expected from Mr. Gilbert's analysis but not 

 found in the actual arid bad-land climate? In the eastern and 

 well-watered part of our country, I have often seen clay-banks 

 much more sharply cut than the equally barren surface of the 

 western bad lands; but even on clay-banks, the minute divides 

 between the innumerable little valleys are not knife-edge sharp; 

 they are rounded when closely looked at. Perhaps they are 

 sharper in wet weather and duller in dry spells. 



If rainfall remain constant and structure vary, then the harder 

 the structure, the less the supply of soil for creeping and the 

 sharper the divides; the weaker the structure, the more plentiful 

 the supply of soil for creeping and the duller the divides. Numer- 

 ous examples of this variation might be given. 



LETTERS TO THE EDITOR. 



#•* Correspondents are requested to be as brief a.s possible. Tlie wriier's name 

 is in all cases required as proof of good faith. 



On requfst in advance, one hundred copies of the nutnt/er containing his 

 communication will be fumislied free to any correspondent. 



The editor inillbe glad to publish any queries consonant with tht^ character 

 of the journal. 



Some Remarks on the Botanic Trinomial. 

 An article in Science for September 16, signed C. H. Tyler 

 Townsend, contains certain statements which cannot be passed, 

 it seems to me, without some few words of discussion. It is quite 

 evident that this article loses sight entirely of the main purpose 

 of a biological name, and seems to imply that the name of a thing 

 lias to do with justice, right, etc. For example, I find therein 

 the following expressions: "In no case can the name of the 

 original erector and describer of a genus be separated therefrom 

 without gross injustice." "There is no necessity whatever for 

 shedding glory upon the one who has made the transfer. . . . He 

 has no right whatever to the species." These words, "injus- 

 tice," "right,"' belong to the field nf Ethics, not that of Tax- 

 onomy. 



