188 



LYELL'S ELEMENTS OF GEOLOGY. 



Cretaceous Coral Reef. 



is very rare in the chalk. Nevertheless wood is sometimes met 

 with, and in the same parts of the chalk where the pebbles are 

 found, both in soft stone and in a silicified state in flints. In 

 these cases it has often every appearance of having been floated 

 from a distance, being usually perforated by boring- shells, such 

 as the Teredo and Fistulana.* 



The only other mode of transport which suggests itself is sea- 

 weed. Dr. Beck informs me, that in the Lym-Fiord, in Jutland, 

 the Fiicus vesiculosus sometimes grows to the height of ten 

 feet, and the branches rising from a single root, form a cluster 

 several feet in diameter. When the bladders are distended, the 

 plant becomes so buoyant as to float up loose stones several 

 inches in diameter, and these are often thrown by the waves 

 high up on the beach. The Fucus giganteus, of Solander, so 

 common in Terra del Fuego, is said by Captain Cook to obtain 

 the length of 360 feet, although the stem is not much thicker 

 than a man's thumb. It is often met with floating at sea, with 

 shells attached, several hundred miles from the spots where it 

 grew. Some of these plants, says Mr. Darwin, were found ad- 

 hering to large loose stones in the inland channels of Terra del 

 Fuego, during the voyage of the Beagle in 1834; and that so 

 firmly, that the stones were drawn up from the bottom into the 

 boat, although so heavy that they could scarcely be lifted in by 

 one person.f Some fossil sea-weeds have been found in the 

 cretaceous formation, but none, as yet, of large size. 



Cretaceous coral reef in Denmark. — Having said so much 

 on the probable derivation of chalk from the decay of corals 

 and shells, I may add, that in the island of Seeland, in Denmark, 

 there is a yellow limestone intimately connected with the chalk, 

 and containing a vast number of the same fossils, which consists 

 of an aggregate of corals, retaining their forms as distinctly as 

 the dead zoophytes which enter into the structure of reefs now 

 growing in the sea. The thickness of this rock is unknown, but 

 it has been quarried at Faxoe to the depth of forty feet. At 

 Stevensklint, in Seeland, it is seen to rest on white chalk with 

 flints, from which it diflfers greatly in appearance, and where it 

 is covered again by another limestone, which although of later 

 date, agrees more nearly with the white chalk, both in fossils and 

 mineral character. Out of 104 species oflfeponges, corals, and 

 other zoophytes, collected from the limestone of Faxoe, and from 

 the ordinary white chalk of Denmark, which agrees with that of 

 England, no less than forty-two are common to both formations ; 

 and many of the same species of bivalve shells and echinoder- 



* Mantell, Geol. of S. E. of England, p. 96. 



t Darwin, p. 303. 



