R. 8. Tarr — Phenomenon of Rifting in Granite. 267 



energy. But allotropic silver also shows an almost exactly 

 similar capacity to respond to the influence of energy in all its 

 manifestations by undergoing changes of a like character. 



The inferences to be drawn from the foregoing seem to be 

 as follows. That silver may exist in three forms : 1st. Allo- 

 tropic silver which is protean in its nature ; may be soluble or 

 insoluble in water, may be yellow, red, blue or green, or may 

 have almost any color, but in all its insoluble varieties always 

 exhibits plasticity, that is, if brushed in a pasty state upon a 

 smooth surface its particles dry in optical contact and with 

 brilliant metallic luster. It is chemically active. 2d. The in- 

 termediate form, which may be yellow or green, always shows 

 metallic luster, but is never plastic and is almost as indifferent 

 chemically as white silver. 3d. Ordinary silver. . . . Further, 

 that allotropic silver can always be converted, either into the 

 intermediate form, or directly into ordinary silver ; that the 

 intermediate form can always be converted into ordinary silver, 

 but that these processes can never be reversed, so that to pass 

 from ordinary silver to allotropic it must first be rendered 

 atomic by combination, and then be brought back to the 

 metallic form under conditions which check the atoms in unit- 

 ing. That allotropic silver is affected by all forms of energy, 

 and that this effect is always in one direction, namely, towards 

 condensation. That the silver haloids are similarly affected by 

 the same agencies. That a remarkable parallelism is notice- 

 able between the two actions, especially if we take into account 

 that in the haloids the influence of energy is to some extent 

 restrained by the strong affinity which the halogens show for 

 atomic silver. There is therefore reasonable ground to sup- 

 pose that in the silver haloids silver may exist in the allotropic 

 form. 



Philadelphia, March. 1891. 



Aet. XXIX. — The Phenomenon of Rifting in Granite; by 

 Ealph S. Tarr/ 



[Published by permission of the Director of the U. S. Geological Survey.] 



In the granite at Cape Ann and elsewhere it is noticed that 

 the rock splits most easily in certain fixed directions; and it 

 is by taking advantage of these lines of weakness, that large 

 regular blocks are easily split from their bed in the quarry. 

 An expert quarryman knows full well just what may be ex- 

 pected of the granite and in making his calculation, the prime 

 factor is the direction and strength of the u rift." In many 

 places there are other lines of weakness along which the 



