202 GRANITES AND GRANITIC VEIN-STONES. [XI. 



granite of Fariolo to a process of shrinking, and a subsequent 

 segregation filling the resulting cavities, in which he is forced 

 to recognize the intervention of water, though by no means ad- 

 mitting the aqueous origin of veins, since he holds even those 

 of quartz to have been formed by igneous injection. (Geologie 

 Lyonnaise, *278.) 



26. When we consider the cause which has produced the 

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

 which hold the granitic veins already described, it is to be re- 

 marked that their comparative abundance, their shortness and 

 their irregularity, distinguish them from the fissures which are 

 filled with eruptive rocks. Examples of the latter may be seen 

 near Danville, Maine, where dikes of fine-grained dolerite are 

 posterior to the endogenous granitic veins here occurring in the 

 mica-schist. These dikes may be supposed to be dependent 

 upon movements in the earth's crust opening deep fissures 

 which connected with some softened rock far below. Through 

 such openings were extravasated the exotic rocks, whether 

 granites or dolerites, more or less homogeneous mixtures, 

 often widely different in composition from the encasing rocks. 

 The endogenous veins, on the contrary, are distinguished not 

 only by their more or less heterogeneous and often banded 

 structure, but by the fact that their principal constituents are 

 generally the mineral species common in the adjacent strata. 



27. Volger has attributed the formation of the openings 

 containing concretionary veins to the force of crystallization, 

 which is shown to be very great in the congelation of water 

 and the crystallizing of salts in cavities and fissures. Such a 

 process once commenced in an opening in a rock would, he 

 conceived, be sufficient to make still wider the fissure, which 

 might be fed by fresh solutions passing by capillarity through 

 the pores of the rock. If this process were to become concen- 

 trated around several points, the intermediate spaces might be 

 so opened that free crystallization could g& on, resulting in the 

 production of goedes in veins thus formed. 



Fournet, on the other hand, suggests that contraction in 

 the cooling of erupted granites gave origin to the fissures and 



