LITHOLOGY. 91 



or elsewhere, a fragment should "be detached from a part 

 that has been least subjected to the action of the 

 weather ; for the composition of a " weathered " surface 

 may have been very materially modified. Some rocks 

 will be thus changed into a substance totally different 

 from their original state, and all are in colour and hard- 

 ness more or less thereby affected. 



Having selected a suitable portion of the rock, let a 

 good-sized piece be broken off by chisel or hammer ; this 

 can afterwards be reduced its most characteristic-look- 

 ing part being chosen into a fair hand specimen. In 

 using the hammer for "stone-breaking," it should be 

 borne in mind that the fracture will follow, as nearly as is 

 possible, the line of the blow, the force of the descending 

 implement (itself stopped by contact) passing on in the 

 same direction, unless turned aside by the mass being 

 too rigid to be broken through. A fissile rock may thus 

 be split up into slabs by blows on the edge ; a piece 

 broken off at right angles to the bedding by a direct 

 blow on the surface ; and a projecting corner may be 

 chipped off a very hard and solid rock by well-directed 

 blows, when otherwise fragments are unobtainable. It 

 is also worthy of remembrance that one good swinging 

 blow is worth a dozen minor taps, and the risk is no 

 greater of a resulting fracture to the hammer-handle. 



The rock to be determined should be broken, with as 

 little chipping as may be, into a square fragment, not a 

 rounded lump, the larger surfaces representing the lines 

 of bedding, with the others at right angles or there- 

 at p. 221, representing the planes of cleavage and original stratifi- 

 cation. 



