ON TEE FORMATION OF CRYSTALLINE ROCKS. 293 



In the silicions rocks quartz has been isoLated in a great variety of 

 forms. The granitic rocks and some porphyries contain it in grains 

 or in isolated crystals. In the schistose rocks it is sometimes nnder 

 the form of veins or thin flakes, lodged between the lamillre, with an 

 uniformity of thickness and parallelism that is surprising, as in mica, 

 chlorite, and talcose schist, leptinites, phyllades, &c., sometimes 

 under the form of veins which distinctly cut the lamellae at the same 

 time that it attaches itself to them. Sometimes, even, quartz in a 

 granular state constitutes considerable masses, as in the rocks 

 (itacolumites) of Brazil, which are associated with gold and diamonds. 

 In the greater number of cases, however, quartz appears to be the 

 product of a decomposition of silicates, just as in my own experiments, 

 where it was produced in different ways. Thus quartz, which under 

 so many forms constitutes a part of eruptive and metamorphic rocks, 

 should be considered equally with that of veins as a witness for the 

 wet way.* 



We may conclude from what happens in the experiments on super- 

 heated water, as from the example of limestones so charged with 

 minerals, which are thrown out from the interior of Mount Somma, 

 that heat and pressure appear to be indispensable to the production of 

 an energetic metamorphism. On the other hand, an intense metamor- 

 phism has been sometimes developed near the surface, as in Brazil, 

 where the crystalline schists containing gems extend more than 200 

 miles in length. . There seems to be a contradiction between these 

 two facts, and yet, when the superheated M'ater is impelled from 

 great depths toward the surface through the substance or the scarcely 

 opened fissures of a rock, it must be remarked that the laws of hydros- 

 tatic pressure are not applicable to it, as it would be to water ascend- 

 ing freely in a crevasse. We can easily understand that in the first 

 case its pressure, and consequently its temperature, might be pre- 

 served, as it were, in a close vessel, up to only a few feet from the 

 surfiice. It is therefore possible that many phenomena, such as the 

 crystallization of granite and of certain masses of tin, which contain 

 the same minerals as the rocks of Brazil, may have taken place under 

 pressure, although at no great depth. 



It is possible that some minerals, the anhydrous silicates, for 

 instance, are not easily produced in water except at determinate tem- 

 peratures. Too much heat, as well as a want of heat, is prejudicial 

 to their formation. Moreover, experiment seems to show that the 

 feldspars are sometimes produced and sometimes destroyed in water, 

 according to the temperature. It is probable, that because in some parts 

 of the Alps, such as the Grisons, the higher portions alone furnished 

 a proper temperature, that metamorphism and the various minerals 

 which are, as it were, witnesses, have been produced there rather than 

 in the beds situated lower down, whose section we can follow in the 



* My experiments thus fully ccnfimi the viyws which Schafhaiit and Bischof have 

 published on this subject. 



