116 LITHOLOGICAL GEOLOGY. 



with much difficulty. In New York State it was necessary — first to 

 study all the sections in the eastern, central, and western parts, and 

 determine carefully the fossils in each stratum ; then to compare 

 the sections with one another : when any case of identity in the 

 fossils among these strata of the different sections was observed, 

 it was set down as one horizon determined. By this method, and 

 other aid from observing the continuity of beds, one horizon after 

 another was ascertained, and the strata between were arranged 

 according to their true order of succession. 



There are precautions required in the use of this key, depending 

 on individual differences in the continents and diversities in the 

 range of fossils, which will be better understood after a review of N 

 the general progress of life on the globe. 



126. Subdivision into Ages. — By the means explained, great 

 progress has been made in arranging the rocks of the different 

 continents in a chronological series. North America has some 

 large blanks in the series, which in Europe are filled ; and in this 

 way various countries are contributing to its perfection. This 

 series has been divided into Ages, based on the progress of life, 

 as follow : — 



I. Azoic Age (from a, privative, and C,uov, animal).- — Containing no 

 traces of animal life. 



II. Silurian Age, or Age of Mollusks. — Mollusks the dominant 

 race. 



III. Devonian Age, or Age of Fishes. — Fishes the dominant 

 race. 



IV. Carboniferous Age, or Age of Acrogens. — Characterized by 

 coal-plants, or Acrogens. 



V. Eeptilian Age. — Reptiles the dominant race. 

 VI. Mammalian Age. — Mammals the dominant race. 

 VII. The Age of Man. 

 The subdivisions are given beyond. 



127. Thickness of the stratified rocks. — The whole thickness of 

 the rocks in the series has been stated at fifteen or sixteen miles. 

 But this includes the sum of the whole grouped in one pile. As 

 the series is nowhere complete, this cannot be said to be the thick- 

 ness observed in any one region. The rocks of New York, down 

 to the Azoic, counting all as one series, are about 13,000 feet in 

 thickness. They include only the Silurian and Devonian (except- 

 ing the Triassic in the southeast). To the north they thin out 

 to a few feet, while they thicken southward towards Pennsyl- 

 vania. In Pennsylvania the rocks include the Carboniferous, 

 and the whole thickness is at least 40,000 feet. This is exclusive 



