Geology. 



417 



This result, combined with another obtained by Mr. Hopkins, proves 

 that so great pressure and friction exist at the surface of contact of the 

 shell and nucleus as to cause both to rotate together nearly as one solid 

 mass. Other grounds for believing in the existence of the great pressure 

 exercised by the nucleus at the surface of the shell were adduced. If the 

 density of the fluid strata were due to the pressures they support, and if 

 the earth solidified without any change of state in the solidifying fluid, 

 the pressure against the inner surface of the shell would be that due to 

 the density of the surface stratum of the nucleus, and would, therefore, 

 rapidly increase with the thickness of the shell. Contraction in volume 

 of the fluid on entering the solid state would diminish this pressure, but 

 yet it may continue to be very considerable, as the co-efficient of con- 

 traction would always approach towards unity. The phenomena of the 

 solidification of lava and of volcanic bombs were referred to in illustration 

 of these views, and their application was then shown to some of the 

 greatest questions of geology. The relations of symmetry which the re- 

 searches of M. Elie de Beaumont seem to establish between the great 

 lines of elevation which traverse the surface of the earth appear to Prof. 

 Hennessy far more simply and satisfactorily explained by the expansive 

 tendency of the nucleus which produces the great pressure against the 

 shell than by the collapse and subsidences of the latter. The direction of 

 the forces which would tend to produce a rupture from the purely eleva- 

 tory action of the pressure referred to would be far more favorable to 

 symmetry than if the shell were undergoing a distortion of shape from 

 collapsing inwards. The nearly spherical shape of the shell would also 

 greatly increase its resistance to forces acting perpendicularly to its sur- 

 face, so as to cause it to subside, while the action of elevatory forces 

 would not be resisted in the same manner. 



4. On the Great Pterygotus [Seraphim) of Scotland, and other Species ; 

 by Mr. J. W. Salter, (Proc. Brit. Assoc., August, 1856 ; Ath. No. 1504.) 

 This paper was in some measure a continuation of one published in the 

 Quarterly Geological Journal for 1855, describing some new and large 

 Crustacean forms from the uppermost Silurian rocks of the south of Scot- 

 land. They were described under the name of Himanthopterus, and 

 were supposed to differ from the published fragments of the great Ptery- 

 gotus by the lateral position of the large simple eyes. In the general 

 shape of the body, however, the terminal joints and tail, in the want of 

 appendages to the abdomen, as well as in the form and number of the 

 swimming feet, mandibles, maxillae and antennas, there was found to be 

 on further examination the closest resemblance between Himanthopterus 

 and the great Pterygotus. And the resemblance has been carried still 

 further by the favorable collocation of all the known specimens from the 

 Scotch collections which have furnished us with nearly all the portions, 

 and also, with the head, which we now find to be exactly like that of 

 Himanthopterus, having lateral, not subcentral, eyes, as represented by 

 other authors. The two genera are therefore identical, and the group, as 

 now constituted, includes a number both of small and moderate-sized 

 Crustacea, along with some which were far larger than any living species, 

 and which certainly attained a length of six or eight feet ! The collec- 

 tions made by the Scotch geologists, in connection with other specimens 



SECOND SERIES, VOL. XXII, NO. 66. — NOV., 1866. 



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