GEOLOGICAL CONCLUSIONS. 385 
CHAPTER VI. 
GEOLOGICAL CONCLUSIONS. 
THE geological bearing of the facts that have been detailed 
in the preceding pages may have been already perceived by 
our readers. A brief review of the points of more special in- 
terest may serve as a convenient recapitulation of the subject. 
I. FORMATION OF LIMESTONES. 
Coral reefs are beds of limestone made of corals, with the 
help of shells and other calcareous relics of the life of the 
sea. The mode of formation is essentially the same, which- 
ever of the two kinds of organic products — corals or shells 
— predominate ; although in one case the bed would be called 
coral limestone, and in the other, shell limestone. 
The reefs illustrate two different modes of origin of such 
beds: (1), by undisturbed growth, with only additions of fine 
material to fill up the intervals; (2), by the grinding of the 
corals, etc., to fragments, sand, or mud, through the agency 
of the waves. 
Beds made by the former method, have many open spaces 
between the grouped masses or branches, and could not be 
turned into a solid layer of limestone, if situated too deep in 
the ocean to feel sensibly the movement of the waves,—unless 
Rhizopods, or minute shells of some kinds, multiplied so rap- 
idly over the same sea-bottom, as to fill up the interstices. 
There is no reason to believe that such aid from shells or 
