64 FRACTURE OF FLINT. 



or grayish, green, except where the surface has been acted 

 upon, and the average composition of which is 25 parts 

 quartz and 75 felspar. Its physical characters are absence 

 of toughness, and the existence of a splintery conchoidal 



fracture almost as sharp as that of flint At the 



hornblendic extreme of the trap rocks we find the basalt, of 

 which also celts were made ; tough and heavy, the siliceous 

 varieties having a splintery fracture, but never affording so 



cutting an edge as the former Intermediate in 



character between these two rocks, we find all the varieties of 

 felstone, slate and porphyry streaked with hornblende, from 

 which the great majority of the foregoing implements have 

 been made." 



On the whole, however, flint appears to have been the 

 stone most often used in Europe, and it has had a much 

 more important influence on our civilisation than is generally 

 supposed. Savages value it on account of its hardness 

 and mode of fracture, which is such that, with practice, a 

 good sound block can be chipped into almost any form 

 that may be required. If we take a rounded hammer, and 

 strike with it on a flat surface of flint, a conoidal fracture 

 is produced ; the size of which depends, in a great measure, 

 on the form of the hammer. The surface of fracture is 

 propagated downwards through the flint, in a diverging 

 direction, and thus embraces a cone, whose apex is at the 

 point struck by the hammer, and which can afterwards be 

 chipped out of the mass. Flint cones, formed in this way, 

 may sometimes be found in heaps of stones broken up to 

 mend the roads, and have doubtless often been mistaken for 

 casts of fossil shells. 



If a blow is given, not on a flat surface, but at the angle 

 of a more or less square flint, the fracture is at first semi- 

 conoidal or nearly so, but after expanding for a short dis- 

 tance, it becomes flat, and may be propagated through a 



