194 GEOLOGY [CHAP. IX 



Letter 532 hard, and got your Silurian subject well in your head, to 

 have profited by so short an excursion. How I should have 

 enjoyed to have followed you about the coral-limestone. I 

 once was close to Wenlock, 1 something such as you describe, 

 and made a rough drawing, I remember, of the masses of 

 coral. But the degree in which the whole mass was regularly 

 stratified, and the quantity of mud, made me think that the 

 reefs could never have been like those in the Pacific, but 

 that they most resembled those on the east coast of Africa, 

 which seem (from charts and descriptions) to confine extensive 

 flats and mangrove swamps with mud, or like some imperfect 

 ones about the West India Islands, within the reefs of which 

 there are large swamps. All the reefs I have myself seen 

 could be associated only with nearly pure calcareous rocks. 

 I have received a description of a reef lying some way off 

 the coast near Belize (terra firmd], where a thick bed of 

 mud seems to have invaded and covered a coral reef, leaving 

 but very few islets yet free from it. But I can give you no 

 precise information without my notes (even if then) on these 

 heads. . . . 



Bermuda differs much from any other island I am 

 acquainted with. At first sight of a chart it resembles an 

 atoll ; but it differs from this structure essentially in the 

 gently shelving bottom of the sea all round to some distance ; 

 in the absence of the defined circular reefs, and, as a conse- 

 quence, of the defined central pool or lagoon ; and lastly, in 

 the height of the land. Bermuda seems to be an irregular, 

 circular, flat bank, encrusted with knolls and reefs of coral, 

 with land formed on one side. This land seems once to have 

 been more extensive, as on some parts of the bank farthest 

 removed from the island there are little pinnacles of rock of 

 the same nature as that of the high larger islands. I cannot 

 pretend to form any precise notion how the foundation of so 

 anomalous an island has been produced, but its whole history 

 must be very different from that of the atolls of the Indian 

 and Pacific oceans though, as I have said, at first glance of 

 the charts there is a considerable resemblance. 



1 The Wenlock limestone (Silurian) contains an abundance of corals. 

 " The rock seems indeed to have been formed in part by massive sheets 

 and bunches of coral" (Geikie, Text-book of Geology, 1882, p. 678). 



