1897,] Fossils and Fossilization. 31 
ocean basin as a red or gray clay, while in places the siliceous 
parts of radiolarians also furnish a very considerable propor- 
tion of this mineral sediment, and the mass holding carbonic 
acid in solution has doubtless a solvent influence on many of 
the contained testaceous remains, and destroys their perfection 
as fossils. Upon elevation and consolidation into stony layers, 
the process of crystallization, started in the calcareous paste or 
jelly—which process partakes also of the nature of a hardening 
in a natural cement—produces sometimes a cementation of the 
parts, so that the fossils are coherent throughout with their 
matrix, and are extracted with difficulty, or, indeed, but ob- 
scurely detected at all. Centers of crystallization also form in 
the centers of the fossils themselves, by which all trace of 
organic structure is obliterated. . 
One of the most typical and important groups of fossils is 
the corals, and to discover the circumstances of their accumu- 
lation in the past we must look at the coral making portions 
of our globe to-day. Many of the deep sea corals are simple 
or single individuals, and are living in neighborhoods in the 
deep seas, while the great reef-making corals rising in coral 
banks to the surface of the water and prolonged by branching 
or acervuline growths are communal, and these coral colonies 
form the substantial basis of sea islands. They furnish the 
material which is heaped up in calcareous sand strata, making 
porous limestones, such as are seen in the Aeolian rocks of the 
Bermudas, or which, dissolved as a calcareous glue, unites the 
agglomerated fragments of beach shells into the Coquina beds 
of Florida. The coral colonies begin their growth at depths 
hardly exceeding 50 fathoms, though the Challenger explora- 
tions revealed coral life at depths of 1,300 fathoms. If they 
establish themselves in more shallow water at the customarily 
assumed limit of 20 fathoms, the sinking of the shores they 
skirt, according to the convenient hypothesis of Darwin and 
Dana, depresses the platforms from which they start to this 
depth or much more. The thermal conditions probably de- 
termine the depths at which reef-building corals can live, and 
it is a possible and probable circumstance that in varying 
positions and in other geological times, reef-making corals may 
