354 PRACTICAL LESSONS IN SCIENCE. 



very, very long. It is said that no group of strata has been 

 traced over a wider area of the earth's surf ace, and none are more 

 uniform and well marked in their composition and in the charac- 

 ter of their fossil remains than are the Silurian. 



Following the Silurian age came the Devonian age, character- 

 ized by the " old red sandstone beds " of Great Britain. In North 

 America the lowest formation is the corniferous limestone, one of 

 the great limestones of the continent; the later strata were 

 mainly sandstones and shales in the east, with shales and 

 thin limestones in the west. The corniferous limestone is distin- 

 guished by seams of horns tone or flint and occurs in New York, 

 Ohio, Michigan, Iowa, Missouri, Kentucky and adjacent regions. 

 The Hamilton shales of New York and the black shales of Ohio 

 and neighboring regions belong to this age. 



During the Devonian age the different classes of invertebrate 

 life were abundantly represented, but in general by different spe- 

 cies from those of the silurian age. Fishes, however, were the 

 characteristic and dominant life of the age. They were like the 

 sharks and skates, garfish and sturgeons of the present, repre- 

 senting the lower orders of vertebrate life. They existed in great 

 variety, and often attained a large size. 



Plants were represented by seaweeds and other aquatic forms, 

 but terrestrial forms, as equisetaB, ferns, lycopods, and the lower 

 gymnosperms were abundant and characteristic, often attaining 

 the size of trees, forming forests. The corniferous limestone 

 indicates a quiet sea for a long time, but the later rocks were 

 largely the products of erosion deposited on the sea bottom near 

 the coasts. And the vegetation suggests extensive swamps and 

 marshes. The character and relations of the rocks indicate no 

 marked distinction or gap between the Silurian and Devonian 

 ages, but the marked difference between the life forms of the two 

 ages does suggest an interval of time not shown by the rocks. 



The Devonian passed without violent changes into the carbon- 

 iferous age. The rocks in many localities show changes of level and 

 crumpling of strata indicating some difference of conditions be- 

 tween the Devonian and the following Carboniferous age; but the 



