88 PROP. J, LOGAN LOBLEY, F.G.S., ETC., ON 



Avhich was rounded by the rolling action of sea-waves, 

 exactly as is going on now at Brighton. 



In the rocks that were formed after the Cambrian, namely, 

 the Lower Silurian, we find remains of higher jMollusca 

 than those in the Cambrian rocks, for there are fossil 

 gasteropods, pteropods, and cephalopods, some in great 

 abundance. The trilobites and brachiopods of the 

 Cambrians become more numerous and more differentiated 

 so as to give many genera of these groups. The Lower 

 Silurian rocks include limestones, slates, and shales, telling 

 of tranquil waters; and thick masses of volcanic rocks, 

 both consolidated fragmentary ejectamenta, or ashes, and 

 compact basalts, telling of violent eruptions and great lava 

 flows, such as we have at present. 



The inorganic conditions, therefore, of the Cambrian and 

 Lower Silurian epochs cannot be said to have been greatly 

 dissimilar to those of the present time, and consequently we 

 must look chiefly to the organic worlds of plants and 

 animals to find the changes that chiefly prepared the earth 

 for man's existence and abode. 



A great step in this Avonderful progressive march is 

 indicated by the fossils of the next great division of the 

 sedimentary rocks, the Upper Silurian, for these rocks 

 reveal the appearance on the globe of the highest sub- 

 kingdom of animals; — the Vertebrata, since in them 

 are the remains of fishes — true fishes certainly, but of an 

 early type only. Their most conspicuous difference from 

 the usual fishes of the present seas is the form and 

 character of the tail, to the extremity of which the back- 

 bone extended, giving it a prolonged pointed form. These 

 herocercal-tailed fishes, as they are called, have still many 

 representatives in the sharks, sturgeons, and skates, but 

 they are now less numerous than the homocercal or equal- 

 lobed-tail fishes which are now so common and abundant. 

 In Silurian rocks, too, are remains of undoubted land plants, 

 which Sir William Dawson described as Prototaxites, but 

 now called Nematophyton^ which was probably a thallo- 

 phytalform. 



The Devonian epoch saw a considerable development of 

 Vertebrata, as the fishes were numerous, and though 

 retaining the vertebral tail, diverged from the pristine type 

 and approximated to the modern salmon-like f(U-m. Large 

 crustaceans very much larger than our present largest 

 sjjecies, the giant Australian crab, abounded. One of these, 



