71 



sharks and plate-covered ganoids, which probably had no 

 great beauty of color. Their strange forms, however, and 

 their frequently formidable size, made up for their lack of 

 coloration, and they probably were among the most striking 

 tenants of those waters. Shells were by no means un- 

 common, and though many of the modern brachiopods are 

 dull-colored, it is highly probable that some, at least, of the 

 many Corniferous species, showed tints approximating 

 those of tropical gastropods and lamellibranches of the 

 present day. 



Thus, shut off from the destructive forces of the outer 

 ocean by the Devonian land, which then existed to the east 

 and south-east, this great interior sea was peopled with a 

 multitude of organic forms, and the luxuriance of the life can 

 only be imagined from the results which have been left 

 behind. It is an interesting fact that this great Devonian 

 coral reef seems to have been absent from Pennsylvania and 

 the Southern States, and it is possible that this is due to the 

 fact, that the water at that time was deeper over this area. 

 For it is now generally recognized that corals flourish best 

 in comparatively shallow "water, and limestones are no 

 longer regarded as necessarily of deep water origin. It is 

 highly probable that the great Corniferous coral reef was 

 built in shallow water, far enough away from land to be out 

 of reach of the sediment carried down by streams. The reef 

 probably grew southward, where the breakers, rolling in 

 from the open ocean on the south-west, supplied pure water 

 and ample nourishment for the polyps and other organisms. 



How long these conditions continued, is difficult to say. 

 That the time occupied for the growth and accumulation of 

 the Corniferous coral reef was equal in length to that 

 occupied in the accumulation of the much thicker shales 

 succeeding it, will appear, when it is remembered that five to 

 ten feet of fragmental rocks will accumulate during the time 

 required for the formation of one foot of limestone.* We 



*Dana, Man. Geol. 



