18 LOWER OOLITIC ROCKS OF ENGLAND : 
siderable thickness, and being made up of Corals and partly of 
Mollusca, Polyzoa, Echinoderms, Annelides, Sponges, Calcareous 
Alg{E, and Foraminifera, cemented and compacted by infiltration 
of carbonate of lime. Dr. H. B. Guppy says that Coral-rock 
in the Solomon Group is from 70 to 100 feet thick, rarely 150, 
and at most 200 feet.* It corresponds with the limit of depth at 
which corals appear to thrive. 
Among the oolitic rocks we have no reef of any magnitude. 
We have bands made up almost entirely of Corals in the Inferior 
Oolite, in the Great Oolite, and in the Corallian rocks ; but they 
do not extend over large areas, nor are they of great thickness, 
seldom attaining more than 10 feet. Moreover the Coral- beds 
are often more or less earthy, with bands of marls, whereas Coral- 
rock is comparatively pure. Prof. J. F. Blake has remarked that 
the rarity of corals in the Portland rocks may account for the 
scarcity of oolitic beds.f 
Dana remarks that in Coral-reefs " The rock of the outer reef, wherever 
broken, exhibits usually a compact texture. In some parts it consists of 
coral fragments, rounded or angular, of quite large size, firmly cemented. 
Other portions are a finer coral breccia or conglomerate. Still others, 
more common, are solid white limestones, as impalpable and homogeneous 
in texture as the old limestones of our continents. There are also other 
regions where the corals in the rock retain the original position of 
growth." Dr. Guppy also speaks of chalky coral-limestone, like the 
Chalk formation, made up of decomposed coral and calcareous algae. 
Sometimes the rock is magnetdan. 
Dana also says that " The deposits of sand or coral mud over the bottom 
of the seas outside of barrier reefs are sometimes of groat extent. These 
sands are the fine detritus which the return flow of the breakers bears 
seaward ; and, in still deeper water, the deposits should be of the finest 
calcareous sand or mud fit material for impalpable compact limestones. 
The waters outside of the reef, especially when moved by heavy tidal 
currents or storms, are often milky with the coral sand; and while the 
coarser sand is dropped near the shores, the finer may be carried for miles 
and distributed far out to sea."J The formation of oolitic coral-limestone, 
and the deposition of chalky silt over wide areas, has been observed, by 
Prof. A. Agassiz, in his studies of the Tortugas and Florida Reefs. 
In very many respects therefore we find that the chief features 
of our oolitic rocks are repeated in the accumulations connected 
with Coral-reefs. We have the granular and oolitic limestones, 
the pure white limestones, the pebbles or rolled masses of oolite, 
the coral conglomerate or coral rag, &c. ; while the false-bedded 
character of so many of the oolites is seen also in the shelving 
banks of coral-sand and oolitic mud that fringe the Coral-reefs 
and islands. 
Dr. Guppy has noted the presence in the Solomon group of 
beds largely made up of Rhynchonellas, forming a Rhynchonella- 
limestone, such as may be compared with the Brachiopoda-beds 
in the Lower Oolites. Capt. K. J. Nelson observed the occurrence 
* Trans. K. Soc. Edin., rol. xxxii. p. 545. 
t Quart. Journ. Geol. Soc., vol. xxxvi. p. 191. 
j Dana, Corals and Coral Islands, pp. 138, 142, 351 ; and John Murray, Nature, 
Feb. 28, 1889, p. 424. 
Mein. Amer. Acad. Arts & Sci., vol. xi. 1885, pp. 115, 125, 128, &c. 
