FOREIGN COKRESPOXDEXCE. 



281 



The metamorphism of the eruptive rock (whatever be its nature) is 

 generally less characteristic than that of the strata uplifted. M. 

 Delesse thinks this is easy to account for, as the latter were solid at 

 the time the phenomenon took place, and consequently not in a con- 

 dition to exercise a reaction upon the plutonic rock. This is, how- 

 ever, an insufficient reason, and M. Delesse's own observations show that 

 even where no change or metamorphism is apparent in the uplifting rock, 

 a few simple tests will enable us to affirm that a change has really taken 

 place. "When the plutonic rock is examined comparatively (in a large 

 vein, for instance) at its borders and at its centre, it is remarked to 

 have undergone a modification., not only in its structure, but also in 

 its composition. Such modifications do not, however, extend more 

 than a few inches from the borders ; they are more marked in smaller 

 veins, and more visible in lava and traps than in granite rocks. 



Near the borders of a vein of rock its structure has become schistose, 

 prismatic, gTanular, amygdaloi'dal, (fcc, according to circumstances. 

 The density of the rock has diminished in these parts, and this is 

 very notable in the case of trap-rocks. The quantity of water which 

 it contains has, on the contrary, augmented.* In some cases struc- 

 ture alone has been modified ; but in most the composition of the rock 

 is changed also. Sometimes this composition is exactly intermediate 

 between that of the uplifting and that of the upheaved rock. 



Among the minerals found in the eruptive rock near the parts in 

 contact with the uplifted strata^ 31. Delesse indicates carbonates and 

 quartz ; also, difi'erent silicates, principally garnet, ido erase, and 

 epidote. But when the reaction that has taken place between the two 

 rocks has been very active, a complete exchange or mixture of ele- 

 ments has been operated. 



Metalliferous lodes are often seen either in the uplifted or in the 

 plutonic rock. Tiiey penetrate both, and are most abundant at the 

 points of contact. 



As concerns minerals produced during metamorphism by contact, 

 they are very numerous, as we have already shown in our preceding 

 papers, and they are much the same for the plutonic rock and the 

 strata which it has uplifted. In numerous cases these minerals have 

 been formed from the elements of the one and the other. Quartz and 

 spathic carbonates are very frequent where either the eruptive rock 

 or the other contains silica and carbonates. Zeolites are more espe- 

 cially associated with volcanic rocks, such as lava, basalt, and trap. 

 Tourmaline, with granitic rocks. The numerous silicates for which 

 M. Dana formerly established the types garnet and pyroxene have 

 been formed in the eruptive rock, and in the uplifted strata. 



M. Charles Martins, the eminent botanist of Montpellier, informs 

 us that he has discovered, among some notes taken during several 

 scientific excursions made by him at difl:erent times, a striking proof 

 of the correctness of a theory he propounded some time ago to esta- 



* This is probably one of the chief causes of the diminution of specific gravity. 

 — T.L. P. 



