134 AZOIC AGE. 



The Devonian, by heavy vertical lines. 



The Carboniferous, by light cross-lines on a black ground, or by a black 

 surface, or by dots on a black ground (theirs* the Sub-carboniferous, 

 the second the Coal formation, the third the Permian). 



The Reptilian, including the Triassic, Jurassic, and Cretaceous, by 

 lines sloping from the right to the left (/), the Cretaceous being 

 distinguished by having the lines broken. 



The Tertiary, by lines sloping from the left to the right ( \ ). 



The Azoic, by irregular line-dottings. 



The surface without markings is occupied by rocks of undeter- 

 mined age, that on the east mostly crystalline. 



I. AZOIC TIME OR AGE. 



Reality of the Age. — The Azoic age is the age in the earth's his- 

 tory preceding the appearance of animal life. The fact of the 

 existence of the globe at one time in a state of universal fusion 

 is placed beyond reasonable doubt. And whatever events occurred 

 upon the globe from the era of the elevated temperature necessary 

 to fusion, down to the time when the climate and waters had become 

 fitted for animal life, are events in the Azoic age. The age must, 

 therefore, stand as the first in geological history, whether science can 

 point out unquestionably the rocks of that age or not. 



The fossils of true Palaeozoic and Mesozoic rocks have often been obliterated 

 by the crystallization of these rocks ; and, as the oldest rocks of the globe are 

 nearly all crystallized, the question may always arise with respect to any par- 

 ticular one of them, whether it may not once have been filled with fossils. After 

 having reached what was thought to be the lowest fossiliferous beds in Great 

 Britain, fossils have been found in others inferior, carrying the Silurian down 

 to a still lower level by transferring to it what had been regarded as Azoic. 

 Such changes are part of the progress of the science, and cause little inconve- 

 nience to the system, provided the order of succession be rightly given. 



Stratigraphical limits in North America. — The Azoic rocks in 

 North America* at present include all that are older than the Pots- 

 dam sandstone of New York, — the first of the Silurian. This sand- 

 stone is spread out in nearly horizontal layers, conformable with the 

 overlying Silurian beds, but rests on crystalline rocks, which are- 

 upturned at all angles and folded or crumpled on a scale of great 



* The Azoic system of North America was first distinctly recognized in its 

 true importance in the Report of Foster & Whitney on the Lake Superior 

 region. The rocks inferior to the Silurian have been called by Murchison the 

 Bottom rocks. They are part of the 'primary of the old geologists. 



