NA TURE 



[May 4, 1905 



vestigation resolves itself into a study of coloration in the 

 genus." The conditions which make for variation in the 

 different species are well indicated, as the author points out 

 that, even where the inmates of a single nest are examined, 

 the following points have to be considered : — First, that two 

 or three females may work together for the good of one com- 

 munity, and may be very differently coloured ; secondly, that 

 each may be fertilised by several males, which again may be 

 differently coloured ; thirdly, that intruders from other nests 

 may be present as they " are not always so certainly driven 

 away from strange nests as has been affirmed for other social 

 Hymenoptera." 



In these circumstances, the attempt to distinguish the 

 species by colour characters seems to be almost hopeless — 

 a point, however, which seems to the present writer to have 

 been overlooked is the possibility of the presence of un- 

 observed plastic characters which might serve as better and 

 more satisfactory guides to classification. That such char- 

 acters exist among the palcearctic species has been demon- 

 strated by F. F. Kohl in knn. K.K. Naturh. Hofmuseitm, 

 Wien, xiii., heft i., pp. S7-90, taf. iii., who shows that five 

 forms of the males can be easily separated by well-defined 

 characters in the form of the clypeus and genae, the grooves 

 of the face, and the shapes of the subapical joints of the 

 antennjE, and although their respective females and workers 

 have not been satisfactorily identified, it is not improbable 

 that careful investigation may yet disclose characters to 

 associate the sexes of the different species together ; as also it 

 is quite probable that all the species would vary in colour in 

 more or less parallel directions — any investigation into the 

 distribution of the species, unless conducted with special 

 reference to these characters of the males, would be very 

 liable to lead to wrong conclusions. One conclusion especially 

 to which one would like to apply the male character 

 test is summed up in the following words : — " It is hardly 

 probable that we have in P. variatus a primitive species which 

 has differentiated in two directions, but, as we shall see 

 from the study of the geographical distribution of the species, 

 aurifer and pallipes are two originally distinct species which, 

 from the course of their migration northwards, have come 

 together in the Mississippi valley, and by their commingling 

 produced a species having, in some measure, the characters 

 of both." These remarks are made with no wish to depre- 

 ciate, even if it were possible, this very careful attempt to in- 

 vestigate a most difficult problem, but merely to point out 

 that there are characters in our Palsarctic species of Polistes 

 which might be well looked for in those of the other hemi- 

 sphere. 



THE CLEAVAGE OF SLATES. 



'T'HE memoir described below' contains an account of 

 experiments undertaken to test the author's theory, 

 propounded some years ago, of the cause of the cleavage 

 property in slates. Dr. Becker's theory, substantially the 

 same as that put forward earlier by the Rev. O. Fisher, 

 is that cleavage-planes are planes of maximum tangential 

 strain, or in other words shear-planes. This is opposed 

 to the theory of Sharpe (or, as we might say, of .Sharpe 

 and Sorby), which makes the cleavage-planes perpendicular 

 to the maximum compression. The author has misunder- 

 stood Dr. Sorby 's position, having apparently overlooked 

 the earlier papers of that writer. The question whether 

 heterogeneity in the rock is necessary for the production of 

 cleavage ■ seems to be beside the mark, since all rocks 

 (other than glasses) are heterogeneous in this sense. Both 

 Tyndall's wax and Dr. Becker's ceresin, being crystalline 

 bodies, are heterogeneous, and their behaviour must depend 

 on the orientation of the minute component crystals. 



The experiments described were carried out with ceresin, 

 a substance of the paraffin series, and some also with clay. 

 These were submitted in one series of tests to simple com- 

 pression, and in another series to shearing by means of a 

 machine devised for the purpose. In the small masses 

 dealt with the strains developed vary greatly from point 

 to point, and the resulting structure is of a complex kind. 

 We must confess that we are not convinced that the effects 



1 " Experiments on Schisto^ity and Slaty Cleavage." By George F. 

 B-cker. Pp. 34:7 plates. BiiN. No. =41 of U.S. Geological Survey. 

 Washington, 1.0 04.) 



NO 1853. VOL. 72] 



observed are such as to be rightly described as cleavage — 

 they have rather the character of fractures, depending on 

 the application of the forces which produce them, as well 

 as on the intimate structure of the material. 



It is unfortunate that no attempt is made to collate the 

 results of the experiments with actual examples of cleaved 

 rocks. As the author remarks, the position of the strain- 

 ellipsoid affords a crucial test. On the Sharpe-Sorby theory 

 the principal diametral plane of the ellipsoid must coincide 

 with the cleavage-plane ; on Dr. Becker's hypothesis it 

 should be inclined at some angle of less than 45°. Now 

 there are many slates in which the strain-ellipsoid is 

 actually presented in deformed spherical concretions or 

 colour-spots. The " birdseye " slate of Westmorland and 

 the green-spotted purple slates of Llanberis are examples 

 familiar to every English geologist. In every case the 

 orientation of the ellipsoid is that which agrees with the 

 received theory. iVIoreover, the spots are elliptic in the 

 cleavage-plane itself, being elongated, as Dr. Sorby pointed 

 out fifty years ago, in the line of cleavage-dip. If the 

 cleavage-plane were a plane of shearing, it would corre- 

 spond with a circular section of the ellipsoid. 



We might object further that, since there are two direc- 

 tions of circular section, or of shearing, there should, on I 

 Dr. Becker's hypothesis, be always two directions of) 

 cleavage, perpendicular to one another with incipient ( 

 cleavage and making an acute angle in well cleaved slates. 

 Our author endeavours to meet this difficulty in discussing 

 his shearing experiments. One direction of shearing is 

 parallel to a fixed face of the block undergoing deform- 

 ation, while the other is continually changing, " so that 

 any one set of particles undergoes maximum tangential 

 strain along these planes only for an infinitesimal time." 

 Even assuming such conditions to be realised in nature, 

 which cannot be the general case, we should still suppose J 

 that the cleavage-property (as distinguished from fractures! 

 set up in the process of deforination) will depend on the ' 

 actual structure of the rock, not on the manner in which 

 that structure has been arrived at. 



It will be apparent from the foregoing criticism that, 

 while recognising the intrinsic value of these experiments 

 and the clear manner in which the author's views are set 

 forth, we do not find in them anything which assails 

 successfully the generally accepted interpretation of the 

 cleavage structure. .\. H. 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE. 



Cambridge. — By direction of the Board of Geographical 

 .Studies, part ii. of the examination for the diploma in 

 geography will be held on June 21 and two following days. 

 No person is qualified for admission to part ii. who has not 

 previously passed part i. (or the special exainination in 

 geography for the ordinary B..'\. degree). The names of 

 intending candidates, together with the subjects they pro- 

 pose to take up, should be notified to the registrary not 

 later than May 24. The fee for admission to the examin- 

 ation is, for members of the university, 3L ; for persons 

 not members of the university, 5Z. The fee must be paid 

 to the registrary not later than June 15. The subjects are 

 regional geography, surveying and mapping, geomorph- 

 ology, oceanography and climatology, the history of geo- 

 graphy and anthropogeography. Copies of the schedules 

 defining the range of examination may be obtained by 

 application at the registry. 



The council of the Senate has recominended that the 

 University of Queen's College, Kingston. Ontario, be 

 adopted as an institution afifiliated to Cambridge University. 



It is understood that the syndicate for considering the 

 studies and examinations of the university, the report of 

 which in favour of the abolition of compulsory Greek in 

 the previous examination was thrown out last term, will 

 continue to meet. It is proposed to add to the syndicate 

 Mr. E. S. Roberts, master of Gonville and Caius College ; 

 Dr. Adam, one of the tutors of Eminanuel College ; Sir. 

 S. H. Butcher, late professor of Greek at Edinburgh 

 University ; and Mr. G. H. Hardy, of Trinity College. 

 These gentlemen were on the " non-placet " side at the 



