ON THE FORMATION OF CAVITIES IN ROCKS. 19 



are usually covered with a layer of serpentine or talc or some 

 such secondary product. The strain caused by the movement may 

 in rare instances leave the slips in such a state of tension that 

 when, from any cause such as excavation, the pressure is relieved, 

 they will scale off with a small explosion. (See A. Strahan, " On 

 Explosive Slickensides," Geological Magazine, iv. 401, 522.) Ob- 

 servations on the directions of slips may, in cases of doubt, throw 

 some additional light on the direction of the movement which 

 caused the fault. But the best guide in stratified rocks is a knowl- 

 edge of the succession of the beds as revealed by drill cores or ex- 

 cavations. Attempts have been made to deduce mathematical 

 formulas for the calculation of the amount of downthrow or up- 

 throw, and when sufficient data are available, as is often the case 

 in coal seams, this may be done. The methods depend on the 

 projection of the planes in a drawing, on the principles of analytical 

 geometry, and on the calculation of the displacements by means of 

 spherical trigonometry. (See G. Koehler, Die Storungen der Gdnge, 

 Plotze und Lager, Leipzig, 1886 ; William Engelmann. A transla- 

 tion by TV". B. Phillips, entitled, "Irregularities of Lodes, Veins, 

 and Beds," appeared in the Engineering and Mining Journal, June 

 .25, 1887, p. 454, and July 2, 1887, p. 4. A very excellent paper, 

 having a quite complete bibliography, is F. T. Freeland's " Fault 

 Rules," M. E., June, 1892.) Prof. Hans Hoefer has called attention 

 to the fact that in faulting there is frequently a greater displace- 

 ment in one portion of the fissure than in a neighboring part, and 

 ven a difference of hade. This causes a twisting or circular move- 

 ment of one wall on the other, and needs to be allowed for in some 

 -calculations. ( Oestereich. Zeitschrift fur Berg- und Huettenwesen, 

 Vol. XXIX. An abstract in English is given by R. TV. Raymond, 

 M. E., February, 1882.) In the Engineering and Mining Journal 

 for April and May, 1892, a quite extended discussion of faults by 

 several prominent American mining engineers and geologists is 

 given, apropos of the question raised by Mr. J. A. Church as to 

 whether fissure veins are more regular on the dip or on the strike. 

 In a relatively uniform massive rock the regularity should be 

 greater on the dip, but in inclined and diversified stratified rocks 

 too many variables enter to warrant any sweeping assertions. In 

 soft rocks like shales the fissure may become so split into small 

 stringers as to be valueless. Again, in very firm rock, where there 

 is little drawing apart, the fissure may be very tight. In the veins 

 of Newman Hill, near Rico, Colo, (see 2.09.10), the fissure is so 



/^fc- 



f( UNIT*' H 



