22 Prof, llcuucssy on tke Thickneas and 



and occur only at slight elevations above the level of the sea. 

 We were told that the same remark holds good in reference to 

 certain other deposits containing shells, which we did not ex- 

 amine, in the north-eastern extremity of the island, still further 

 from the Peak. 



In the first of the passages above cited. Prof. Smyth has alluded 

 to fossiliferous strata in the islands of Grand Canary and Palma. 

 In regard to Palma, I may mention that Mr. Hartung and I, 

 when we were there in 1854, searched in vain for fossils; no 

 travellers had then found any ; and our correspondents in the 

 Canaries have still no knowledge of any having been obtained iu 

 that member of the Arcliipelago. 



Lastly, as to the Grand Canary, Von Buch was, I believe, 

 the first to call attention to the existence of marine shells in 

 that island, where Mr. Hartung and I collected them in abun- 

 dance in 1854, and ascertained that they are imbedded in nearly 

 horizontal strata continuous over a large area, where they form an 

 elevated platform about 400 feet high, near the town of Las 

 Palmas, a platform terminating abruptly in a range of cliffs near 

 the sea, facing the north-east. These upraised sedimentaiy 

 strata, with some intercalated basaltic beds, are far removed 

 from the slopes of the great dome-shaped volcanic mass, which 

 forms the central nucleus of the Grand Canary; and if they 

 have any bearing on the question of " Craters of Elevation," 

 they certainly do not corroborate that hypothesis, but, on the 

 contrary, are directly opposed to it ; for though they have been 

 upheaved in a district where intermittent volcanic action has 

 never ceased, they do not dip away in all directions from a 

 central axis, nor have they assumed a conical or dome-like form. 



IV. On the Thickness and Structure of the Earth's Crust. By 

 Henry Hennessy, F.R.S., Professor of Natural Philosophy in 

 the Catholic University of Ireland. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



I HAVE read with interest a letter addressed to you by Arch- 

 deacon Pratt, virhich has appeared in your Number for last 

 May. In a problem so extremely difficult and obscure as the 

 determination of the thickness of the solidified crust of the earth, 

 I am not surprised that some differences of opinion should exist. 

 The views which I have published on this subject have been cri- 

 ticized by Archdeacon Pratt on two grounds : — 1st, on account of 

 the assumption that the shell is sufficiently rigid to resist, without 

 change of form, the i)ressure upon its inner surface which arises 



