GEOLOGICAL CONCLUSIONS. 303 



and yet trituration by the action of the waves and winds has 

 in many places reduced all to the finest material, so that an 

 embedded shell is seldom to be found in the beach or drift 

 oolite, and rarely too in much of the fine-grained coral 

 reef-rock. 



The lagoon basin appears to be eminently the place for 

 making these non-fossiliferous limestones. This is the case 

 in two widely different conditions : first, over the portions that 

 are below the coral-growing depths, which are sometimes of 

 great area ; and secofid, in lagoons that have become so small 

 and shallow that corals and large shells have all disappeared, 

 and the trituration is of the finest kind, producing calcareous 

 mud; such lagoons being properly in a marsh condition. 

 These last appear to illustrate on a small scale the conditions 

 under which many of the ancient non-fossiliferous, or sparingly 

 fossiliferous, limestones were formed. 



VII. THE WIDE RANGE OF THE OLDER LIMESTONES NOT EXEMPLI- 

 FIED AMONG MODERN CORAL-REEF FORMATIONS. 



Coral-reefs, though they may stretch along a coast for scores 

 of miles, are seldom a single mile in width at the surface ; and 

 if elevated above the sea, they would stand as broad ramparts 

 separated by passages mostly 20 to 200 feet deep, and often 

 of great width. The substratum, however, is, in general, continu- 

 ous coral-rock ; and if these more elevated parts were removed 

 by any process, after an elevation, they would leave a nearly 

 level area of coral limestone often as extensive as the whole 

 reef-grounds. This is at once seen from the map of the islands 

 of the Gilbert Group (p. 132), or that of the Feejees. In an 

 island like Dean's, one of the Paumotus, these reef-grounds 

 are 1,000 square miles in extent. 



But the most extensive reef-grounds of the oceans are after 

 all of small breadth compared with many of the ancient lime- 

 stones of the continents ; and the reef-rocks also are peculiar 

 in their very abrupt limits, the margins sometimes descending 

 at a steep angle a thousand feet or more. These difierences 



