ON THE GKOLOGY OF BAUBADUS. 223 



From these data we have ventured to draw a section across the 

 eastern part of the ridge and to assume that this part of it at any 

 rate is based upon an isolated bank of the Oceanic deposits (see 

 fig. 9). 



(g) Summdrif of Condiisions. — From the foregoing account of the 

 raised reefs our conclusions may be summarized as follows : — 



1. The thickness of reef-rock formed in a stationary or rising 

 area does not often exceed 200 feet, but may be as much as 2G0. 

 AV'e must not, however, infer from this that the corals began to 

 grow in 40 fathoms of water, because there is always a certain 

 thickness of coral-mud, sand, or breccia at the base of the reef. 



2. Masses of reef-coral have been found in wells down to at 

 least 130 feet, so that we can infer these have grown in 22 fathoms 

 of water, a conclusion which agrees with that obtained by a study 

 of the recent reefs. 



3. The detrital rock at the base of the old reefs seems to havo 

 been accumulated on a slope outside an older reef. Its thickness 

 varies from 1 foot to 40 or 50, and probably even more in some 

 cases, depending doubtless on the steepness of the underlying slope. 



4. The reefs appear to have been formed during successive 

 periods of rest or very slow upheaval, following shorter periods of 

 more rapid upheaval, the movements being similar to those which 

 have affected the west coast of South America. 



5. Each movement of upheaval brought up a lower slope 

 within the limit of coral-growth, and sometimes a submarine ridge 

 like that of Christchurch parish, which became an independent site 

 of coral -growth. 



6. The rock of the higher terraces is greatly altered by water, 

 some beds being coma cted into a hard and heavy white limestone, 

 others being rendered loose and rubbly by abstraction of carbonate 

 of lime. 



(h) Summary of Varieties of Coral-rock in Barbados. — From the 

 notes we took in the field, from the information and specimens 

 furnished by Mr. Easton, and finally from Mr. Hill's observations 

 (which will be found in his 1st Ap])endix to this paper), we think 

 three different kinds or classes of limestone-rocks may be distin- 

 guished among the Barbados reefs. 



1. lieef-rock. — A fairly homogeneous rock consisting of corals 

 and coral-debris compacted together with coral-sand and more or 

 less indurated by infiltration of calcite. 



2. Lar/oo)i and Channel Dei>osits. — These are very various both 

 in regard to component materials and coarseness of grain, but they 

 always include a large proportion of other organisms besides corals, 

 such as raollusca, echinodermata, and foraminifera, and sometimes 

 these shells and their broken fragments make up the mass of the 

 rock. Originally these rocks are of a looser texture than reef-rock, 

 but may become compact by infiltration. 



3. Beach-roclc. — This consists of lumps of coral and reef-rock 

 which may have been torn off the outer reefs and cast up on the 

 beach, together with fragments detached from older raised reefs, 



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