620 



SCIENTIFIC RECREATIONS. 



Fig. 706. Gorgonia gut- 

 tata (natural size). 



the salts of lime extracted from the ocean, but their own dead bodies to form 

 the hard substance of the rock." 



The coral insect is a zoophyte (Anthozoa), which, as 

 may be seen from the illustrations, assumes curious and ele- 

 gant forms, and the coral it produces is a limy or calcareous 

 deposit, which is fixed upon a rocky base. As years go 

 on these accretions become greater and greater, and at 

 length rise above the water. When a little distance below 

 it, the reefs form dangerous and frequently unsuspected 

 barriers, upon which ships are wrecked. The red coral is 

 dredged up from the Mediterranean, where there are exten- 

 sive coral fisheries. This coral is found deep in the water, 

 and never rises to the surface. Formerly there were coral 

 reefs in the European seas, but the changes of temperature 

 stopped their production. The " atolls," or circular coral 

 reefs with an opening at one side, have been described by 

 Professor Darwin. " Who," 

 says the great naturalist, 

 " would not be struck with 

 wonder and admiration on 

 catching sight for the first time of this vast 

 ring of coral rock, often many miles in dia- 

 meter ? Sometimes a low green island is seen 

 beyond it, with a shore of dazzling whiteness ; 

 outside is the foaming surf of the ocean, and within it a broad expanse of 



tranquil water, of pale green colour and ex- 

 quisite purity." These " atolls" mark the situa- 

 tion of sunken islands, and the extension of 

 them and the barrier reefs would seem to indi- 

 cate a slow but decided sinking of the bottom 

 of the Indian and other oceans ; but the " reefs" 

 tell us that the land to which they are attached 

 has not become depressed, and may have be- 

 come elevated, We may then conclude that 

 a continual rising and depression of the land 

 is taking place in various oceans, indicating a 

 sinking of the ocean bed in one locality and 

 the result of volcanic activity in another, for no 

 active volcanoes are found in the regions of depression. 



We must now leave the sea and come to land again, to consider volcanoes 

 and volcanic action there. 



Fig. 707. -Coral {RIadrepora brncJiiata). 



Fig. 708. Spicules cf Gorgonia (magnified). 



VOLCANOES AND EARTHQUAKES. 

 The various phenomena of volcanoes form a subject very difficult to be 



