188 T. S. Hunt—No*£s on Granitic Rocks. 



ing to their opposite faces crystalline plates of albite, whicli are 

 larger than the planes to which they are attached. The cm 

 tals of orthoclase moreover frequently present hollowed-out or 

 hopper-shaped faces, which Fournet happily describes as result- 

 ing from the forming of the frame-work or skeleton of the crys- 

 tals, when the material was not sufficient for their completina 

 A process analogous to this is often seen in crystallization, 

 whether from fusion, solution or vaporous condensation, gi\'iDg 

 rise in some cases to external depressions and in others to inter- 

 nal cavities in the resulting crystals. Fournet ascribes the for- 

 mation of the geodes of the granite of Fariolo to a process oi 

 shrinking and a subsequent segregation filling the resulting cav- 

 ities, in which he is forced to recognize the intervention of 

 water, though by no means admitting the aqueous origin of 

 veins, since he holds even those of quartz to have been formed 

 bv igneous injection. {Oeologie Lyonnaise, *278). 



§ 26. When we consider the cause which has produced the 

 fissures in the mica-schists and gneisses of New England, whicj 

 hold the granitic veins already described, it is to be remarked 

 that their comparative abundance, their shortness and their 

 irregularity distinguish them from the fissures which are Un 

 with eruptive rocks. Exarni )les of tlie Litter may be seen near 

 Danville, Maine, where (lyk(«s of Hne-graincd dolerite are pos- 

 terior to the endogenous '-runitic veins here occurring m the 

 mica-schist. These dykes may b(^ supi)osed to be dependent 



upon movements m the earth s crust openmg f^eep u=--, 

 which connected with some softened rock far below. ihroiV 

 such openings were extravasated the exotic rocks, whether gran^ 

 ites or dolerites,— more or less homogeneous mixtures, o 

 widely different in composition from the encasing rocks, i 

 endogenous veins, on the contrary, are distinguished not o 

 by their more or less heterogeneous and often banded strucnuj 

 but by the fact that their principal constituents are the mine 

 species most common in the adjacent strata. -^ 



§ 27. Volger has attributed the formation of the of |^. J 

 containing concretionary veins to the force of crystall^^t;^^;; 

 which is shown to be very great in the congelation ot^< ^ 

 and the crystallizing of salts in cavities and fissures. ^^^^^^_ 

 process once commenced in an opening in a rock ^^^}r\ a^h 

 ceived, be sufficient to make still wider the fissure, wbicti " .^^ 

 be fed by fresh solutions passing bv capillarity ^^^"^f^^^. 

 pores of the rock. If this process were to become conce^ ^ 

 ted around several points, the intermediate space mig"^ ,|.^ 

 opened that free crvstullization .'ould <io on, resnltm? " 

 production of ^i^vAv> in v.mih thn- rni'i'inMl. 



Fournet, on the other hand ^ui!-<i-<"-;t^ that coiitriU'tion 



