458 
JOURNAL OP THE PLYMOUTH INSTITUTION. 
Fragments of coral 
Portions of encrinites . 
Organic clay 
Crystallized calc spar . 
12-3 
11-5 
25-7 
50-5 
100- 
After long ages of sedimentation and alternating volcanic action 
the bed of the ancient sea whereon we now live became tenanted by 
coral animals, and the formation of a coral reef began. Our lime- 
stone has every characteristic of a fringing reef ; that is, of a reef 
following the main outlines of the shore at no very great distance 
from the land. How the reef-builders were introduced we cannot 
say. One thing is certain, the temperature of this old-world sea 
must have been warmer than that of our present waters. It must 
likewise have been shallow. Reef-building corals do not live in 
ordinary cases at a greater depth than twenty or thirty fathoms, 
rarely below fifteen. Hence there is evidence of great changes. 
Either the palaeozoic sea had become shallower, because of the 
long-continued process of deposition, which is hardly likely to 
have been an exclusive cause, or its bed had been raised by forces 
from within. This is most probable, especially as we have so soon 
to call into play a return movement. The apparent thickness of 
our limestone, taking its breadth and average dip, is about 600 
yards. There may be undulations which would greatly reduce 
this thickness ; but I do not think so. 
In any case the thickness is greater than the natural range of 
the building action of the coral animals ; and therefore, to account 
for its formation, we must call in aid the process now in operation 
in the Pacific. Only a gradual and steady sinking of the land 
would enable the coral animals to build a reef of such extent. In 
a coral reef precisely analogous to those which now exist in the 
Southern Seas the Plymouth limestones then had their origin. 
But it may be asked, Whence the bedding, which indicates 
mechanical and not organic action ? The answer is easy. In a 
coral reef growth and destruction are ever present. No kind of 
live coral reaches above a few feet in height, and the waves and 
winds are unceasingly grinding it down. The matter removed is 
deposited in and around the reef itself, consolidating and extend- 
ing ; and since, while our reef was subject to this double process, 
the ordinary form of deposition continued in the sea around, we 
