ANNIVERSARY ADDRESS OF THE PRESIDENT. CXI 



has in another pubHcation entered very largely on the consideration of 

 this question. Mr. Henry Clifton Sorby has published in the Philo- 

 sophical Magazine a valuable paper on slaty cleavage, as exhibited in 

 the Devonian limestones of Devonshire. The question which he 

 proposes for consideration is, whether slaty cleavage be the result of 

 crystalline or mechanical action ; and he observes that a most careful 

 examination of these rocks, which he considers as peculiarly suitable 

 for the purpose, in the field, microscopically, chemically, and by 

 means of the polariscope, has convinced him that the structure on 

 which their slaty cleavage depends may be completely explained on 

 mechanical principles. The author's arguments are well put to- 

 gether, and with regard to many supposed cases mathematically 

 correct ; but whether applicable to all cases of cleavage is perhaps 

 more than can fairly be assumed, nor does it appear to be a necessary 

 consequence, that because a rock has been exposed to enormous press- 

 ure, its surface must therefore have spread out over a more extended 

 space in proportion to that pressure, as the author seems to assume 

 in the account of his practical experiment with the pipe-clay, and 

 the scales of oxide of iron. The author concludes by observing that 

 the cleavage of the limestones varies directly as the amount of 

 mechanical compression to which they have been subjected, and 

 that he cannot therefore hesitate to conclude that slaty cleavage is 

 the result of mechanical, and not of crystalline forces. 



Amongst the works of a more general character, I must remind 

 you of the publication of Sir Charles Ly ell's last edition (fifth) of 

 the ' Manual of Elementary Geology,' or the ancient changes of the 

 earth and its inhabitants, as illustrated by geological records. It is 

 distinguished by much additional information, derived both from the 

 extended observations of the author himself, and from the labours 

 and researches of other geologists. A most interesting chapter in 

 this volume is the detailed account of the Canary Islands, and the 

 celebrated Island of Palma, regarded by the late Leopold von Buch 

 as the type of what he called a " crater of elevation," or " Erhe- 

 bung's Crater." Sir Charles Lyell has recently personally examined 

 these localities, in company with Mr.Hartung of Konigsberg, to whose 

 zeal and talents he was indebted for much assistance, and the result 

 of the examination has been to call in question the "elevation theory'* 

 of the celebrated Prussian geologist. 



After carefully describing the physical features of the district, and 

 the various strata of scoriee, lapilli, and lava which constitute the dif- 

 ferent beds. Sir Charles points out several remarkable circumstances 

 which militate against the elevation-crater theory. He particularly 

 alludes to the absence of fractures in the rim of the great cavity of 

 the Caldera, which must have been the result of any elevatory action. 

 He fairly shows the greater probability of the mass of inclined beds 

 and strata round the orifice being due to the accumulation of ejected 

 tufPs, lapilli, and scoriaceous matter falling round the crater, from 

 which they had been ejected, and thus forming a compact mass of 

 igneous matter of various kinds, sloping in every direction, at a con- 

 siderable angle. 



