METAMORPHISM. 767 



through the action of siliceous waters, has been changed to quartz. Silicified shells and 

 woods are also examples of pseudomorphs. The chemical methods of making pseudo- 

 morphs are very various, and some of the processes are the same in kind with those that 

 have operated on a larger scale in metamorphism. For example, the several minerals, 

 chrysolite, pyroxene, chondrodite, chlorite, which are all anhydrous magnesian silicates, 

 and others, have been changed to the hydrous magnesian silicate, serpentine ; for ser- 

 pentine occurs with the crystalline form of each of these species, and some of the pseu- 

 domorphs observed contain a portion of the original mineral at the centre. Such ser- 

 pentine pseudomorphs are very common and various, and this is one reason for doubting 

 the organic nature of Eozoon (p. 158). As another example, the mineral corundum 

 (crystallized alumina) has been shown by Genth to occur, in western North Carolina, 

 changed, through some way or other, to chlorite, margarite (a kind of mica), cyanite, 

 diaspore, and other mineral species. So pyroxene has been proved to occur altered to 

 talc, the rensselaerite of Fowler, Dekalb, and other places in northern New York, hav- 

 ing in some places the composition of talc, with the cleavage planes of pyroxene. Again, 

 large hexagonal prisms occur at Diana, in Lewis County, N. Y., which consist of the 

 soft hydrous silicate called pinite (resembling the agalmatolite of China, there worked 

 into images and ornaments) which were derived, it is supposed, from the alteration of 

 nephelite (a silicate related in composition to the feldspars). This making of silicates, 

 if only in an isolated way, shows what nature's out-door processes are capable of doing. 

 But these same methods of chemical change have sometimes gone forward on an exten- 

 sive scale sufficient to make rock formations, so as to be true examples of 



Pseudomovphic Metamorphhm. — Chrysolite has been changed to serpentine so exten- 

 sively as to make large beds of it; and so has pyroxene, and perhaps also chondrodite. 

 Distinct altered crystals often occur in such beds, and confirm other evidence as to the 

 origin of the great masses. 



In the rock of the Alps named Euphotide (p. 75), the prominent constituent, called 

 saussurite, in some places shows, by the forms of crystals, that it is altered labradorite; 

 its granular texture and other characters prove it to be throughout of this general na- 

 ture, — an altered feldspar. In some regions portions of the mass are still true labra- 

 dorite. Along with the saussurite occurs the hornblende mineral smaragdite, and this is 

 proved to be in some places pseudomorphous pyroxene. Serpentine in beds often ac- 

 companies the rock, and this sometimes indicates that it may have come from the al- 

 teration of pyroxene. Euphotide is thus an example of pseudomorphic metamorphism. 

 It received a crystalline condition first through ordinary metamorphism, making it a 

 labradorite and pyroxene rock, or labradorite and hornblende rock ; but now in conse- 

 quence of a subsequent change, it is euphotide. (On this subject, see Amer. J. Sci., 

 III., xvi., 340, 1878 ) 



Much of the doleryte of the world, as microscopic examinations by various observers 

 has shown, is chloritic, through the change of much of the pyroxene, with sometimes 

 part of the feldspar (labradorite), to a hydrous silicate of the chlorite group. The forms 

 of pyroxene and labradorite are still more or less distinct, or can be made out through 

 gradations in a series of specimens; but now chlorite constitutes the crystals, or parts 

 of them. Here again is pseudomorphic metamorphism. Other results of the metamor- 

 phic change often exist in the chloritic doleryte which are here passed b} r without 

 notice. 



With regard to the doleryte, it has been stated, on page 748, that the water which pro- 

 duced the change (often making the doleryte also amygdaloidal as well as dull in lus- 

 tre), must have been received into the rock while it was melted and on the way to the 

 surface, and came from subterranean sources among the intercepted strata; for the dis- 

 tance to which moisture can " descend into such rocks from above after the}' are solid 

 and cold " is very small, as is shown by the limited depth to which decomposition goes 

 on; and the inability of this cold water to make chlorite out of pyroxene, is proved by 

 its uniformly oxydizing the iron, if air is present, and by the absence of action if it is 

 not. Analogous changes have taken place in trachyte and phonolyte, filling them with 

 zeolites, and in porphyries, hydrating and making quartz-crj-stals in them. This view 



