22 ROCK-METAMORPHISM. 



edges more or less rounded off, evidently by corrosion. Decal- 

 cification exposes these facts most instructively*. The origin 



the calcite must be obvious to any one who has studied 

 pseudomorphism. 



The same phenomenon is displayed, though under somewhat 

 modified circumstances, in the hemithrene ("calcaire saccha- 

 roide/" Delesse) of certain localities in the Vosges. At St. 

 Philippe, near Ste. Marie aux Mines, this rock is filled with 

 crystalloids of malacolite and other mineral silicates, often 

 almost to the exclusion of calcite. Confining ourselves for the 

 present to the malacolite, its crystalloids, or aggregations of 

 them, are more or less affected by corrosion or decretion, 

 beginning with the rounding-off of their angles and edges; 

 next, reducing the aggregations, it forms them into rude, irre- 

 gular, geniculated shapes; and next etches them into some- 

 what definite configurations foliaceous, dendritic, plumose, 

 radiate, and often beautifully arborescent. The configurations 

 vary much in size, some being observable with a hand mag- 

 nifier, others so minute as to be only made out by a good 

 microscope. Fig. 1, Plate III., represents one of the aggre- 

 gations, which has taken the shape of a branching configura- 

 tion f: its component crystalloids, in a corroded condition, are 

 well seen. 



Guided by a remark made by Delesse, we expressed our 

 suspicion some time ago that the ' ' calcaire saccharoide " of the 

 Vosges would yield on examination these and other structures J; 

 but we had no idea that it was so rich in examples rivalling, and 

 in no way surpassed in beauty and imitativeness, the configura- 

 tions (" canal system viEozoon") which at that time had become 

 known as occurring in Canadian ophite, though since then we 

 have published the fact, previously unknown, that precisely the 



* Heddle mentions what appear to be similar examples, occurring at Muir 

 and Midstrath, in which the "lime is little more than granular malacolite 

 with but little lime between the crystals " (see Trans. Roy. Soc, Edin. vol. 

 xxviii. p. 461). 



t Zirkel has represented a portion of a crystal of mica (fig. 35, p. 87, ' Die 

 mikrosk. Beschaffenheit d. Min. u. Gesteine ') which, through corrosion or 

 decretion, has assumed a dendritic or branching form. This example shows 

 very well how crystals and other mineral bodies have taken the remarkably 

 imitative shapes often displayed by malacolite and serpentine. 



J Quart. Journ. Geol. Soc. vol. xxii. p. 188, footnote. The " nummuline 

 wall " is also present, See Introduction, A.D. 1880. 



