VOLCANISM AND ORE DEPOSITS. 1017 



seen that if gaseous solutions are produced in the belt of cementation, where 

 the opening's are numerous and the rocks are saturated, there is a strong 

 tendency for the gaseous solutions to condense (see pp. 1019-1020). In 

 this we doubtless have the partial explanation of the dominance in the belt 

 of cementation of ore deposits, which show all the evidence of having been 

 deposited by aqueous solutions, although many such ore deposits are con- 

 tiguous to igneous rocks. 



In the zone of flowage the conditions for intrusions and their effects upon 

 ore deposits are very different from those in the zone of fracture. It has 

 been pointed out that in the zone of flowage the igneous rocks make their 

 way by elevating and thrusting the intruded rocks aside, by actually mash- 

 ing- them so as to shorten their diameters at right angles to the intrusiv.es, 

 and finally by breaking across their structures. The effect of the igneous 

 rocks is both direct and indirect, as in the zone of fracture. The direct 

 effect is the heating of the surrounding rocks, their absorption to some 

 extent, and occasionally even their local fusion. The indirect effect is of 

 far greater consequence. In this deep-seated zone, where the amount of 

 water is small and the openings are usually small and discontinuous, it is 

 highly probable that the solutions are largely gaseous. In all but the 

 upper parts of this zone the natural increase of temperature, due to depth 

 alone, is sufficient to raise the solutions above the critical temperature of 

 water, and thus produce gaseous solutions. Where igneous rocks are 

 present in large quantity and contribute heat it is little short of certain 

 that gaseous solutions are abundant. Therefore it is believed that gaseous 

 solutions, comparatively unimportant in the zone of fracture, are of great 

 importance in the zone of flowage (see pp. 1020-1021). 



CIRCULATION AND WORK OF SOLUTIONS. 



Solutions in rocks occur in two forms, gaseous solutions and aqueous 

 solutions. Where the underground solutions are referred to without quali- 

 fication there is no implication as to their state, and they may be as either 

 one or the other, or partly both. Oftentimes for aqueous solutions the term 

 water is used, but in such usag-e it is not meant to imply that the water is 

 pure. It has been shown (pp. 146-152) that the general forces producing 

 circulation of solutions are gravity, heat, mechanical action, and molecular 

 attraction. These forces act very differently and do not have the same 



