ON THE GEOLOGY OF BAEBADOS. 223 



Pliocene fossils at a height of 150 feet along the line of rail from 

 Panama. In Nicaragua there seems to be a nearly complete series of 

 Tertiary deposits flanking the central range of highlands/ including 

 beds which are referred to the Pliocene. 



[We are informed by Mr. John Hughes, P.C.S., that deposits of 

 white siliceous and calcareous earth occupy considerable areas in 

 Guatemala. He has given us a sample of the former which proves 

 to consist entirely of diatoms and sponge-spicules. 



Extensive deposits of diatomaceous earth have recentlj" been dis- 

 covered in South California. They are described by l)r. A. M. 

 Edwards,^ of Newark, U.S.A., who informs us that they extend from 

 Monterey, south of San Erancisco, where they are associated with 

 foraminiferal marls, to beyond San Pedro. The diatoms in them 

 are similar to those in the celebrated liichmond earth, and it is 

 quite possible that they are of Pliocene age, though hitherto referred 

 to the Miocene.] 



Although more evidence is required, all that is known favours 

 the conclusion that Central America is now a very much higher and 

 more connected tract of land than it was at any period of Tertiary 

 time and that there has been upheaval during Pleistocene time. 



It is satisfactory to find that on this point we are really in accord 

 with Prof. A. Agassiz. In Part I. of this memoir we quoted certain 

 passages from ' The Three Cruises of the Blake,' the meaning of 

 which we took to be that in Prof. Agassiz's opinion the complete 

 separation of the Atlantic and Pacific Oceans by the elevation of the 

 Isthmus took place at the close of Cretaceous time. Erom a letter 

 with which he has recently favoured us we find that we mis- 

 understood him, and what he meant to convey was that a freer com- 

 munication between the two oceans had existed in Cretaceous than 

 in early Tertiary time, and that the ridging up of the sea-floor which 

 eventually led to their separation began in Eocene time. Mention- 

 ing the Cretaceous deposits of the southern part of the Isthmus, he 

 writes : " I merely assumed that this connection was quite extensive 

 and must have continued from that time more or less uninter- 

 ruptedly nearly to the present time, and that during that time the 

 Gulf Stream must have passed over the Isthmus of Panama, or a 



great part of it, into the Pacific I concluded in a general way 



that the separation of Atlantic and Pacific commenced at some time 

 late in Cretaceous or early in Tertiary time, and was absolute only 

 in the latest Tertiary or comparatively recently. The district 

 affected extended from the Isthmus of Tehuantepec to and including 

 the northern part of South America (Colombia), but I had no 

 intention of stating that the separation was complete at first." 



We have quoted Prof. Agassiz's letter because on re-perusing the 

 passages referred to we still think they seem to bear the construction 

 we put upon them, and it is therefore the more needful that the 

 author's meaning should be explained. 



The opinion has been expressed that unless the Isthmus was very 



1 J. Crawford, Brit. Assne. Eep. 1890, p. 812, 



2 Am. Jouru. Sci. vol. xlii. (1891) p. 309. 



