56 Professor Horsford on the 



Egypt, since man inhabited that almost holy region, and 

 erected in it some of his earliest monuments.* But how 

 long will it be before we shall be able to calculate backwards 

 by our finite measure of time, to those remote periods, in 

 which some of the greatest physical features of this con- 

 tinent were impressed upon it, when the lofty mountains 

 from which the Nile flows were elevated, and when the 

 centre of Africa (certainly all its southern portion) was a 

 great lacustrine jungle, inhabited by the Dicynodon and 

 other lost races of animals? — (Vide Address at the Anni- 

 versary Meeting of the Royal Geographical Society, 1852.) 



Solidification of the Rocks of the Florida Reefs, and the 



Sources of Lime in the Growth of Corals. By Professor 



Horsford, of Harvard. 



I. It is required to ascertain by what processes, chemical 

 or mechanical, or both chemical and mechanical, the surface 

 and the submerged coral rocks have become hardened. 



By the surface rock is intended that thin brown crust, com- 

 posed of numerous layers, which is distinguished by great 

 compactness, and a peculiar ring, when, in detached condi- 

 tion, it is struck by a hammer, and which occurs on the ab- 

 rupt ocean side, and more abundantly on the long slopes on 

 the land side of the Keys. 



By the submerged rock, is intended the rock of oolitic ap- 

 pearance which has solidified under water, and which is of 

 inferior hardness to the surface rock. 



The surface rock, so called, has, in many place?, no longer 

 the outermost position, though it had at the time of its for- 

 mation. It is, indeed, interstratified with friable light colour- 

 ed limestone. The epithet indicates the circumstances of its 

 formation, not its present position. 



* See the account of the instructive suggestions of my friend Mr Leonard 

 Horner, to ascertain the amount of the successive deposits in the Lower Valley 

 of the Nile, as given in Jameson's Edinburgh Philosophical Journal of July 

 1850. Mr Horner informs me that the researches are now going on vigorously 

 on the site of Memphis, having been already applied to the site of Heliopolis, 

 our Consul-General in Egypt, the Hon. C. Murray, taking a lively interest in 

 their progress. 



