ANNIVERSARY ADDRESS OF THE PRESIDENT. CXxix 



formation. In noticing this paper, and alluding also to that by Mr. 

 Biuney, I may observe that much of the extension of the Permian 

 has been obtained by the abstraction of portions of strata formerly 

 allotted to the New lied Sandstone ; and that, whilst the continuance 

 and still powerful development of the genera Productus and Sjnrifer 

 approximates the Permian closely to the Carboniferous system, it 

 may be added, that the mineral connexion is also striking, as mag- 

 nesian limestone occurs extensively in the carboniferous limestone ; 

 for example, as a member of the carboniferous series at the Hook, 

 and again near Cork, where it has given rise to a flourishing manu- 

 facture of carbonate of magnesia. 



In respect to papers bearing on the physical forces which act upon 

 the earth's crust, I may notice a brief communication in the Philoso- 

 phical Magazine, by Professor Hennessy, of the Catholic University 

 of Ireland, on the Influence of the Earth's Internal Structure on the 

 Length of the Day. He adopts the theory of a solidified crust 

 having a still fluid nucleus within it ; and, rejecting Poisson's view of 

 cooling from the centre to the surface, he assumes that the crust, 

 in passing through the vitreous to the crystalline condition, has con- 

 tracted within itself, so as to leave a space between it and the 

 nucleus, or, in other words, so as to diminish the pressure upon that 

 nucleus, which would of course then expand and fill up the space 

 with a semi-fluid body of a diminished density : the cooling then 

 proceeding, new layers would be added to the interior of the crust, 

 which, contracting by crystallizatiofi, would again remove the press- 

 ure on the nucleus, and lead again to its expansion. Mr. W. Hop- 

 kins had before proved, from astronomical considerations, that the 

 crust and nucleus m.ust move together nearly as if they formed one 

 body, and Prof. Hennessy adds, that there must be great friction 

 between the nucleus and the interior of the crust, which should 

 be taken into account when considering the effect which would follow 

 any sudden fracture of the crust. This distinction betvv'een a con- 

 traction of the crust within itself from crystallization, and the general 

 or cubical contraction of the v.hole mass on cooling, leads to a differ- 

 ence between the deductions of Professor Hennessv and those of M. 

 Delesse, who concludes that the whole diameter of the earth would 

 be diminished, and the velocity of rotation increased ; whereas, by the 

 removal, as it were, of the matter from the centre to the surface by 

 its crystallization and adherence to the inner surface of the earth, 

 the moments of inertia would be augmented, and the length of the 

 day increased. Mr. Hennessy considers that the contraction of the 

 general mass is in this way counterbalanced, and that the length of 

 the day remains, and will for ages remain, palpably unaltered. 



These glimpses at the internal condition of the earth are interest- 

 ing and important as regards the operations of igneous rocks ; but, 

 however ingenious the reasonings upon the subject may be, the data 

 are as yet too speculative to admit of any positive certainty in the 

 geological deductions dependent upon them. 



The recent operations of the National Survey in Scotland have also 

 an interesting bearing on the condition of the earth's crust, and I 



VOL. XIII. i 



