(■) MARKS OF rERCUSRION ON SILKEOL'S ROCKS, 



struck oflf a parent block before the desired one was ob- 

 tained. We have therefore 



.Flake 



T, X 1 1 1 / Plane of 



I'arent block r , 



\. nacture 



"^ Nucleus 



We will now study the effects of this blow on the 

 parent block; there are only two alternatives: either a 

 llake was detached, or it was not. If the blow was effec- 

 tive, a flake of smaller or larger size was detached ; but if 

 it was not effective, the result must be a. shattering of tne 

 surface into countless splinters at that point where the ham- 

 mer struck the parent block, viz., the "point of impact." It 

 IS further obvious that in order to detach a flake, the 

 hammer must not penetrate into the matrix of the parent 

 block. If it does, a good deal of the energy will be spent 

 in shattering and jDulverising the matrix, and the re- 

 mainder of the force is probably not sufficient to detach a 

 suitable flake. The hammer must also strike the surface in 

 one point only, and for this reason a spherical hammer or a 

 pebble is the most suitable implement. A flat or pointed 

 liammer will either shatter the surface or penetrate into 

 the matrix. 



If the blew did not detach a flake, that is to say, if it 

 wag ineffective, such I'esult may have been due to insuf- 

 ficient energy, or the penetration of the hammer into the 

 matrix, or both causes. The result will, however, always be 

 the same, viz., a shattering of the surface, and its intensity 

 is determined by the energy of the blow and the resistance 

 of the parent block. 



A.— MARKS OF INEFFECTIVE BLOWS. 



Traces of ineffective blows are frequently observed ; 

 thev are ])articularly common on rejects in the quarries, 

 and Plate I. gives a very good idea of the effects of an in- 

 effective blow. 



The principal result of an inffective blow is the px'a 

 duction of a fairly deep impression or indentation whose 

 surface is broken by numerous fine fissures running more 

 or less parallel ; the fine lamellse of rock thus produced 

 are intensively splintered liy cross fissures, thus producing 



