BY LEO A. COTTON. 513 



4. The great length (more than a mile), and the great uni- 

 formity in both length and breadth, are not favourable to the 

 lateral secretion theory. 



5. The presence of ascending warm water is in favour of a 

 deep-seated origin. 



6. The presence of stannite, which could not have been formed 

 from meteoric waters by any known chemical reactions. 



Sequence of Processes involved in the Formation of the 



Ore-Body. 



The following is put forth as an explanation of the Borah 

 Creek ore-deposits. 



In or about Permian times, the shales in this portion of New 

 England were intruded, first by a biotite granite, and later by 

 the " acid granite." As this last intrusion solidified, it cooled 

 from above downwards, thus forcing the unsolidified portions of 

 the magma — largely silico-aqueous solutions of minerals — to a 

 greater depth in the earth's crust. The contraction of the mass, 

 consequent in cooling, gave rise to strains which were relieved 

 by systems of fracture, more or less constant in direction, along 

 which the aqueous mineral-bearing solutions were able to rise. 



It has been pointed out that there were two eras of fracture 

 at Borah Creek, one contemporaneous with the fractures of the 

 Tingha tin-district, and the other postdating the formation of 

 these ore-deposits. The earlier fracture remained closed until 

 the development of the later one. The later fissure is, by far, 

 the more important, and its formation was attended by the 

 development of a crushed zone along the line of fissure. Solu- 

 tions arising along this crushed zone, attacked the felspars, and 

 eflfected complete kaolinisation. Following this kaolinisation, 

 further movement occurred, giving rise to slickensides on the 

 walls of the crushed zone. The movement was predominantly 

 vertical in some parts, and horizontal in other parts of the mine. 

 Next came the opening of the fissure, which gave a more ready 

 access to the solutions. In the course of this opening, the 

 crushed zone adhered, for the most part, either to one wall or 



38 



