THE VOYAGE OF THE BEAGLE 521 



a course round which horses have been exercised ; hence the 

 hill has been called the Devil's Riding School. I brought away 

 specimens of one of the tufaceous layers of a pinkish colour 

 and it is a most extraordinary fact, that Professor Ehren- 

 berg" finds it almost wholly composed of matter which has 

 been organized : he detects in it some siliceous-shielded fresh- 

 water infusoria, and no less than twenty-five different kinds 

 of the siliceous tissue of plants, chiefly of grasses. From 

 the absence of all carbonaceous matter, Professor Ehrenberg 

 believes that these organic bodies have passed through the 

 volcanic fire, and have been erupted in the state in which 

 we now see them. The appearance of the layers induced me 

 to believe that they had been deposited under water, though 

 from the extreme dryness of the climate I was forced to im- 

 agine, that torrents of rain had probably fallen during some 

 great eruption, and that thus a temporary lake had been 

 formed into which the ashes fell. But it may now be sus- 

 pected that the lake was not a temporary one. Anyhow, we 

 may feel sure, that at some former epoch the climate and 

 productions of Ascension were very different from what 

 th?y now are. Where on the face of the earth can we find 

 a spot, on which close investigation will not discover signs 

 of that endless cycle of change, to which this earth has been, 

 is, and will be subjected? 



On leaving Ascension, we sailed for Bahia, on the coast 

 of Brazil, in order to complete the chronometrical measure- 

 ment of the world. We arrived there on August 1st, and 

 stayed four days, during which I took several long walks. 

 I was glad to find my enjoyment in tropical scenery had not 

 decreased from the want of novelty, even in the slightest 

 degree. The elements of the scenery are so simple, that they 

 are worth mentioning, as a proof on what trifling circum- 

 stances exquisite natural beauty depends. 



The country may be described as a level plain of about 

 three hundred feet in elevation, which in all parts has been 

 worn into flat-bottomed valleys. This structure is remark- 

 able in a granitic land, but is nearly universal in all those 

 softer formations of which plains are usually composed. 

 The whole surface is covered by various kinds of stately 



6 Monats. der Konig. Akad. d. Wiss. zu Berlin. Vom April, 1845. 



