36 



OUTLINES OF GEOLOGY. 



any vegetable life, and does not renew itself. The fossil 

 wood has been partially mineralized by a new combination 

 of its elements. The modern chemical formations occur in 

 the midst of the diluvial, showing that the causes to which 

 the former are due have acted both during the alluvial and 

 diluvial period. 



The beds of mud, clay, marl, and sand, which belong to 

 this diluvial period, are most easily distinguished by their 

 position ; all such as are beyond the reach of existing water- 

 courses must be ranked in this order. 



The gravel and boulders of diluvium are not only distin- 

 guished by position, but by well marked characters. The 

 boulders of diluvium are often much larger, and can be traced 

 to a far more distant locality than those of alluvium, and 

 they, as well as the pebbles of the gravel, are much less 

 worn than are those found in existing water-courses. 



80. Diluvial formations are not only more extensive than 

 any of the alluvial, with the exception of the humus, but are 

 of much greater thickness. Diluvial gravel is sometimes 

 found forming extensive plains, or constituting hills of small 

 elevation. On the island of New- York, the diluvium occu- 

 pied all that portion south of Union Square, rising in some 

 places into hills 80 feet in height, and wells have been sunk 

 in it twenty feet, so that its extreme thickness was more than 

 100 feet. In the valley of the Po and in Provence, the thick- 

 ness of the diluvium is said to amount to 1500 feet. 



81. Among the most remarkable of the phenomena of 

 diluvium are the erratic blocks, or masses of rock rolled by 

 water, which are found mingled with the gravel, or lying 

 scattered on the surface of the ground. These occur singly, 

 but more usually arranged in groups. The instances of er- 

 ratic blocks which have been most carefully studied, are as 

 follows : 



1. The eastern slopes of Jura, a mountain composed of a 



