298 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



about seventy feet. It occurs in the Carboniferous formation, and, like tlie 

 porphyrite of Mosquito gulch, while apparently following a given bedding 

 plane, actually changes from one horizon to another within a vertical range 

 of about seventeen hundred feet. 



It is apparent from the above facts that in underground tlows of igneous 

 rocks, as in lavas flowing on the surface, the coefficient of extent in relation 

 to thickness of flow is a function of the relative basicity and consequent 

 fluidity of the fused mass. 



Amount of intrusive force. — In studying' tlicse intrusivc sheets one is forci- 

 bly impressed with the magnitude of the force exerted during the intrusion 

 of the lava, which here seems almost capable of actual measurement. 

 Assuming as a type of these sheets the great body of White Porphyry 

 above the Blue Limestone, it is seen that at its thickest point under 

 White Ridge it has pried open the strata a distance of about fifteen 

 hundred feet vertically, since that estimated thickness of White Porphyry 

 is now found between the Blue Limestone and the overlying Weber Grits. 

 The fused rock-mass at the time of its eruption must have been nearly fluid 

 enough to obey the laws of hydrostatic pressure, in accordance with which 

 the force applied to it at any point would be equally distributed throughout 

 its mass and equally transmitted in every direction against its boundary 

 walls. As long, therefore, as it retained the fluid condition, this force would 

 be expended, not only in raising the beds immediately over the vent, but 

 in spreading open the strata at as great a distance from this vent as the fluid 

 mass could penetrate. The fluid condition was not, however, retained indefi- 

 nitely, but the mass cooled gradually, and, in cooling, became solid and no 

 longer capable of transmitting the hydrostatic pressure ; therefore, the force 

 available for prying open the strata became gradually less as the distance 

 from the vent increased, and the distance by which the strata were forced 

 apart at the vent may be assumed as the measurement of the maximum force 

 exerted. It has been seen that the thickness of beds above this horizon to 

 the top of the Cretaceous, which is the series assumed to have accumulated 

 before the igneous rocks were injected or intruded, is estimated at 10,000 

 feet. It is possible that at the time of intrusion these beds were still under 

 water, in which case the weight of the water should also be added. But 



