1914] 



Lawson: Ore Deposition by Meteoric Waters 



225 



equally well affect the igneous rock itself ; although it is probable that 

 the fracturing is inaugurated beyond it by collapse due to crystal- 

 lization. Certain veins in volcanic rocks in Nevada, which came under 

 my observation a few years ago, I ascribed in an unpublished report 

 to shrinkage by cooling of an intrusive mass. The temperature of the 

 region in which shrinkage cracks should begin to form would be but 

 little less than the melting-point of the magma, and therefore well 

 above the lower limit of the range specified by Lindgren. The neces- 

 sary conduits and temperature being thus provided, we may proceed 

 with the question of circulation. Referring to the diagram, figure 1, 

 which is a graphic epitome of our assumed conditions, it is apparent 



A 



Fig. 1. Ideal sketch of an intrusive mass of the Monzoni type in stratified 

 rocks which dip toward the intrusion, to explain the possibility of the circula- 

 tion of heated meteoric waters on the periphery and through it. 



that water entering at A and B and following the stratification will 

 tend to keep the lower part of the syncline saturated. The intrusive 

 mass provides conduits transverse to the stratification at C-C, initially 

 by disruption and subsequently by shrinkage, and it also creates a high 

 temperature in the rupture zone. If the temperature were uniform 

 throughout the region included in the diagram, the movement of the 

 water would be up at C on the artesian principle. If the temperature 

 at C be high and that at A be low, the artesian principle operates 

 a fortiori, a short column of superheated steam being lighter than a 

 long column of water. As the steam is drawn off above C the supply 

 is replenished from A and B, so that a prolonged circulation is main- 



