300 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



he shows that the increment of heat and the increment of pressure from 

 the surface toward the interior of the earth are not the same, but may be 

 expressed by two curves which would cross each other at a given depth. 

 Under normal conditions, by the time the temperature in depth has increased 

 to the ordinary fusion point of .J-ock masses, the pressure has also increased 

 to such a degree as to raise the fusion point of these rock masses, so that it 

 is no longer possible for tliem to fuse. This he considers the permanent 

 condition of the earth below the point of junction of the curves of temper- 

 ature and pressure. Now, if for any reason the pressure is suddenly 

 decreased, as it would be by the removal of a considerable weight of rock 

 from the surface over a given area, and if this removal is more rapid than 

 the change of temperature, which owing to the low conductivity of rocks 

 must be very slow, fusion would set in and a subterranean lake of molten 

 rock be formed. He conceives that for n)Ountain areas the removal of 

 large amounts of rock material by erosion would be relatively rapid enough 

 for this purpose. Upon the thus melted mass there would be exerted the 

 pressure of the rocks above it, and probably also an additional pressure due 

 to expansion of its own mass by fusion, which would force the liquid magma 

 toward the surface. 



Why intrusive and not surface flows? The Uext queStlon that SUggestS itself 



is, why did the fused masses which formed the older rocks stop in their up- 

 Avard course at a given horizon and spread out there, instead of continuing 

 on upwards to the surface, as did the more recent flows ? Was it owing to 

 a difference in the chemical composition of the magmas from which either 

 series were formed or to a difi"erence in the quality and amount of the im- 

 pelling force, or, again, to a difference in the resistance offered by the rock 

 masses through which they passed! The first of the three alternatives 

 may, it would seem, be at once answered in the negative, since the same 

 range in ultimate chemical composition is found in intrusive rocks as in 

 recent lavas. The distinction that petrographers have claiined to find be- 

 tween the older or intrusive rocks, as a class, and the recent lavas, depends 

 on internal structure and the arrangement of the mineral constituents, while 

 they acknowledge that the chemical composition of the two classes may be 

 practically identical. In this region White Porphyry and Nevadite among 



