XL] GRANITES AND GRANITIC VEIN-STONES. 203 



geodes now filled or partially filled with crystalline minerals at 

 Eariolo ; we may readily suppose that a process of contraction 

 attendant upon the crystalline aggregation of the materials of 

 sedimentary strata would give rise to rifts or fissures therein. 

 The lesions thus produced in the solid rocks become more or 

 less completely repaired, if we may so speak, by an effusion of 

 mineral matter from the walls, and thus are generated geodes, 

 irregular masses, and many veins. That the process imagined 

 by Volger may in some cases intervene, and may act subse- 

 quently to the one just imagined, is highly probable, though 

 we are disposed to assign it but a secondary place in the pro- 

 duction of vein-fissures. It offers, however, the most plausible 

 explanation of the distortion of the thin-bedded strata already 

 noticed in connection with some of the concretionary granitic 

 veins of Maine, which seem, by a process of growth, to have 

 bent outward the adjacent beds. The vertical transverse veins 

 are, in many cases at least, unsymmetrical, as if they had 

 grown from one side, while the distortion of the beds, some- 

 times attended by irregular concretions in the banded vein- 

 stone, appears at the opposite wall. The notion that the vein- 

 fissures opened as crystallization advanced has been defended 

 by Griiner. 



28. It is not here the place to discuss how far the greater 

 and deeper fissures of the earth are dependent upon the con- 

 traction of sediments, as just explained, or upon the wider 

 spread movements of the earth's crust, though even of these it 

 may be said that they are more or less directly the results of a 

 process of contraction. It should, however, be noted that while 

 some fissures of this kind are filled with dikes of erupted rocks 

 ( 26), others hold concretionary veins, which are to be distin- 

 guished from the class of veins just described, inasmuch as the 

 openings in which they were deposited evidently communicated 

 with the surface of the earth. Examples of these are seen in 

 the lead and zinc-bearing veins with calcite and barytine, which 

 traverse vertically the carboniferous limestone in England, and 

 enclose in their central portions material of liassic age, abound- 

 ing in the remains of a marine and a fresh- water fauna, which 



