PRESENT SURFACE OF NEW YORK 179 



The petrified wood, leaves, moss, etc., which are common in our 

 limestone districts, are of modern date, and are forming at 

 the present time. The rain-water which percolates through the 

 crevices of the limestone rocks, by means of the carbonic acid 

 which it gathers from the air, dissolves the carbonate of lime; and 

 on coming again to the air in springs, re-deposits it in the form 

 of tufa, a drab-colored mass which is nearly pure carbonate of 

 lime. This, as it gradually forms, incrusts the leaves, sticks, etc., 

 with which it comes in contact; and often, as they decay, replaces 

 them in such a manner as to present the same form and structure; 

 pieces of wood being thus replaced by a stony mass closely re- 

 sembling the original substance. 



Age of man 



Man is the most highly specialized member of the animal king- 

 dom. His remains are not found in deposits earlier than the post 

 glacial, which appear to have an age of not many thousand 

 years. There is, so far, no clue to his origin. The first relics 

 of man are rude implements of stone or bone such as knives, 

 arrow-heads, etc., and are found in the gravels of streams and in 

 caves. 



The first period of man is known as the stone age, but though 

 it ceased long ago in Europe, in North America it has existed 

 to within the present century. The bronze age succeeded the 

 stone age. Last of all came the age of iron which is the present. 



PRESENT SURFACE OF NEW YORK 



Under this head it is important to consider briefly the causes 

 which have reduced so large a portion of the rock strata of New 

 York, from the original condition of the wide and uninterrupted 

 extent in which they were formed to that of an undulating and 

 broken aggregate of hills and valleys which we now see. It is 

 probable that during the slow process of emergence from their 

 native sea, the action of waves and currents wore them deeply 



