ZINC ORES OF THE EDWARDS DISTRICT 1 5 



But whether or not this conclusion is right, one thing is clear, 

 namely, that the calcite of the limestone, in its present coarsely 

 crystalline condition, and what may be distinguished as the primary 

 silicates, diopside, tremolite and phlogopite, were formed during this 

 period of metamorphism. They are compounds that are stable 

 under metamorphic conditions of the kind that controlled the 

 recrystallization of the limestone. These conditions gradually 

 changed to others under which the silicates in question were no 

 longer entirely stable and they underwent a partial or complete alter- 

 ation, to establish equilibrium under the new conditions. This alter- 

 ation produced secondary silicates, talc and serpentine, which, 

 together with unchanged portions of the original silicates, have 

 persisted to the present time (in spite of radical change of con- 

 ditions) except at, and close to, the immediate surface, where the 

 agents of weathering have completely broken down the rocks. 



If the varying intensity of the metamorphic processes were 

 represented by a curve, with a vertical coordinate for intensity and 

 horizontal coordinate for time, its summit would indicate the forma- 

 tion of diopside, tremolite and calcite, this aggregate of minerals 

 constituting the typical products of metamorphism. Minerals 

 formed during the period represented by the ascending portion of 

 the curve would tend to be destroyed or recrystallized at its summit, 

 and might or might not be present in the rock now. 



The minerals formed under maximum intensity of metamorphism, 

 at the summit of the curve, are, as a rule, stable through a wide 

 range of conditions and thus persist to a large extent throughout the 

 descending portion of the curve. But even these yield considerably 

 to the new conditions and, thus, the descending portion of the curve 

 represents the alteration of diopside and tremolite into talc, ser- 

 pentine, etc., which, though also silicates, are evidently later than, 

 and derived from, the primary silicates first named. 



This relation between the two sets of silicates is one that has been 

 clearly established in a large number of cases, in many different 

 regions and in rocks of various ages, being perfectly familiar to all 

 students of rocks although, even now, the details of the complex 

 changes involved are obscure. 



In the present case, as in others, microscopic study of thin sec- 

 tions of the rocks shows plainly the time relations of the earlier and 

 later minerals. A granular aggregate of calcite and diopside, in 

 which the two minerals are essentially of the same age, having 

 crystallized in interlocking grains, often shows the diopside in every 



