47§ THE JOURNAL OF GEOLOGY. 



their fine grain and by the perfect preservation of tracks and 

 burrows which were made in soft mud, and of soft animals like 

 jelly fish, that they were deposited in water of considerable 

 depth. The sediment was laid down slowly and gently in water 

 so deep as to be free from disturbance aud under conditions so 

 favorable that it contains the remains of delicate animals not 

 often found as fossils. 



While the fauna of the lower Cambrian undoubtedly lived 

 in water of very considerable depth it was not oceanic but 

 continental, for we are told by Walcott that "one of the most 

 important conclusions is that the fauna of the lower Cambrian 

 lived on the eastern and western shores of a continent that in 

 its general configuration outlines the American continent of 

 to-day." "Strictly speaking the fauna did not live upon the 

 outer shore facing the ocean, but on the shores of interior seas, 

 straits or lagoons that occupied the intervals between the several 

 ridges that ran from the central platform east and west of the 

 main continental land surface of the time." 



This fauna was rich and varied, but it was not self-support- 

 ing, for no fossil plants are found, and the primary food supply 

 was pelagic. Animals adapted for a rapacious life, such as the 

 pteropods, were abundant, and prove the existence of a rich sup- 

 ply of pelagic animals. All the forms known from the fossils 

 are either carnivorous, like the medusae, corals, Crustacea, and 

 trilobites, or they are adapted, like the sponges, brachiopods, 

 and lamellibranchs, for straining minute organisms out of the 

 water or for gathering those which rained down from above, and 

 the conditions under which they lived were very similar to those 

 on the bottom at the present day. 



Walcott's studies show that the earliest known fauna had the 

 following characteristics: It consisted, so far as the record 

 shows, of animals alone, and these were dependent upon the 

 pelagic food supply for support. While small in comparison 

 with many modern animals, they were gigantic compared with 

 primitive pelagic animals. The species were few, but they rep- 

 resent a very wide range of types. All these types have mod- 



