GEOLOGICAL PERIODS. 13 



supply us with our principal fuel; in the Permian, the 

 New Red Sandstone, the Magnesian Limestone (Zechstein), 

 and the Cupriferous Slate were formed. The approxi- 

 mate thickness of this entire group of strata is esti- 

 mated at 42,000 feet at most; some geologists make it 

 eomewhat more, others considerably less. In any case, 

 this Palaeolithic Epoch, taken as a whole, is consider- 

 ably shorter than the Archilithic, but yet is considerably 

 longer than all the following Epochs taken together. The 

 strata deposited during this Primary Epoch supply fossil 

 animal remains in great abundance ; besides numerous 

 species of Invertebrates we find also very many Verte- 

 brates Fishes preponderating. As early as the Devonian, 

 and even during the Carboniferous and the Permian 

 Periods, there existed so great a number of Fishes, espe- 

 cially Primitive Fishes (Sharks) and Ganoids, that we may 

 designate the entire Palaeolithic Period as the Age of Fishes. 

 The Palaeozoic Ganoids especially are represented by a 

 large number of forms. 



But even during this period some Fishes began to 

 accustom themselves to living upon the land, and thus gave 

 rise to the Amphibian class. Even in the carboniferous 

 system we find fossil remains of Amphibia the earliest 

 terrestrial and air-breathing animals. In the Permian 

 Period the variety of these Amphibia becomes greater. To- 

 wards its close the first Amnion-animals, the tribal ancestors 

 of the true higher Vertebrate classes, seem first to appear. 

 These are a few lizard-like animals, of ^ which the Protero- 

 saurus from the Cupriferous Slate at Eisenach is the best 

 known. The appearance of the most ancient Amnion 

 Animals (Amniota), to which the common parent-form of 



