ORGANIC. 265 



warmer latitudes and shallower depths of the ocean. 

 "Within thirty degrees on either side of the equator, and at 

 depths within twenty fathoms, these polypes in numerous 

 genera are perpetually piling up their beautiful calcareous 

 structures ; here encircling lagoons, there fringing islands, 

 and in another area extending in long ridge-like barriers. 

 Many of these coral-reefs are of vast extent the great 

 " Barrier" of ]N"ew Holland being upwards of 1000 miles 

 in length, and from 20 to more than 100 feet in thickness; 

 and in all, the mass is essentially composed of coral 

 structure, but intermingled more or less with shells, crusts, 

 coral-debris, and other extraneous substances. There is 

 nothing more marvellous than this enormous secretion' of 

 rock-matter by the tiniest of agencies ; nothing more over- 

 whelming to the conception than the number of individual 

 organisms concerned in the work ! And yet as it is now, 

 so it has been in all time past ; the same agencies have ever 

 secreted the surplus lime from the ocean-waters, and built 

 their reefs much in the same dimensions and much after the 

 same style of construction. As with coral-reefs, so with 

 serpula-reefs (annelids that secrete calcareous cases); shell- 

 beds^ whether drifted or buried in situ ; bone-shoals drifted 

 by currents, or frequented by fishes, seals, and other creatures 

 that die and accumulate in myriads; and with guano-deposits ', 

 the droppings of sea-fowls that have accumulated under a 

 rainless sky for ages. All are alike of animal origin, and 

 all are alike returning to the sblid crust that which other 

 agencies had dissipated and dissolved. The importance of 

 these organic deposits serpula-reefs 12 or 15 feet thick, 

 like those of Bermuda ; shell-beds many leagues in extent, 

 like those of the Indian Ocean ; bone-shoals like that lying 

 in the North Sea between the Faroe Islands and Iceland ; 

 and guano-deposits 40, 60, or 80 feet thick, like those off 

 the coast of Peru are not sufficiently recognised by geolo- 



