64 



GILPIN ON NOYA 



the ground permitting the formation of veins would be measured by the line where the 

 pressure in the synclinals counterbahvnced the relief afforded by the anticlinal elevations. 

 Whatever approximation theoretical calculations may allow to this line, in view of the 

 thickness of the strata concerned in these movements, it may safely be assumed that it is 

 at a depth beyond the present reach of the miner. Mr. F. B. Bulkely in a paper read be- 

 fore the Philadelphia (1884) meeting of the American Institute of Mining Engineers on 

 the intrusive bedded felsite dykes of Leadville, Colorado, puts this view as follows : — That 

 the application to rock beds of a tangental pressure leads not only to their folding but to 

 their partial separation, and in this connection gravity becomes an important factor. 



The following figure, taken from his paper, represents an ideal anticlinal fold i^ro- 

 duced by the horizontal force PP. Thu.s the resultant P' P" of this force asapplied at 

 O and O', shows an uplift applied equally to all the strata, which is couseqtiently exerted 

 with increasing effect upon the uppermost sheets, causing a tendency to separation of the 

 strata. The force of gravity is represented by G O, G 0", and it is evident that at the 



Fig. — An ideal anticlinal fold. 



srimmit and at the foot of the anticlinal, as at O O this whole force is exerted in a direc- 

 tion at right angles to the bedding planes of the structure, its whole effort being exerted 

 to prevent a separation of the strata, while at other points, midway between the foot and 

 summit, at O", the force thus exerted is much less. An analysis shows that the force 

 exerted at right angles to the plane of the stratification is directly proportional to the 

 cosine of the angle of dip, and is represented by 6' O" 



Stratigraphically, the Nova Scotia veins conform to the conditions implied in such 

 foldings. The present surface of a gold field may be represented as a horizontal section 

 of an anticlinal, in some cases drawn near to its apex, but frequently so low down as to 

 expose nearly all the beds of the gold-bearing horizon. Mining explorations in numerous 

 districts have shown that veins, several inches thick at the surface, gradvially thin and 

 disappear within short vertical distances, that " cross cuts " driven at various depths 

 below the surface have intersected veins which did not outcrop, and that some few veins 

 have maintained nearly uniform dimensions to depths varying from 300 to Ï00 feet, the 

 maximum depths as yet attained in our workings. The lateral thinning out of a vein is 

 often seen to be succeeded by the commencement of another a few inches or feet to one 

 side of the line of its course, as well as the passage more or less abrupt of the plane of a 

 vein through a bed of rock to resume again its normal dip. Similarly, veins have been 

 known to turn almost at right angles across the strata for a distance of a few feet or yards, 



