T. S. Hunt— Notes on Granitic Bocks. 189 



geodes now filled or partially filled with crystalline minerals at 

 Fariolo, and we may readily suppose that a process of contrae- 

 tion attendant upon the crystalline aggregation of the material > 

 of sedimentary strata, would give rise to rifts or fissures there! i 

 The lesions thus produced in the solid rocks become more . 

 less completely repaired, if we may so speak, by an eflfusion > 

 mineral matter from the walls, and thus are generated geoW- 

 irregular masses and many veins. That the process imagiii' 

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

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

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

 duction of vein-fissures. It offers however the most plausil 

 explanation of the distortion of the thin-bedded strata aliH-io , 

 noticed in connection with some of the concretionary gi-aniti. 

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

 bent outward the adjacent beds. The vertical trant^ verse \ cins 

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

 from one side, while the distortion of the beds, sometimes at- 

 tended by irregular concretions in the banded veinstone, appear- 

 at the opposite wall. The notion that the vein-fissures opened 

 as crystalhzation advanced has been defended by Griiner. 



§ 28. It is not here the place to discuss how iar 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 dykes of erupted 

 rocks (§26), others hold concretionary veins, which are to be 

 distinguished from the class of veins just described, inasmuch 

 as the openings in which they were deposited evidently com- 

 municated with the surface of the earth. Examples of these 

 are seen in the lead and zinc-bearing veins with calcite and bary- 

 tine, which traverse vertically the Carboniferous limestone m 

 England, and enclose in their central portions material of lias- 

 sic age, abounding in the remains of a marine and a fi-esh-water 

 fauna, which shows these veins to have been deposited m fis- 

 sures communicating with the surface-waters of the liassic 

 period. For a description of these veins by Mr. Charles Moore, 

 see the Report of the British Association, for 1869, and this 

 Journal, II, 1, 365. Similar evidence is afibrded by the exist- 

 ence of rounded pebbles imbedded in veins, as obser\'ed m 

 Bohemia, and also in Cornwall, where numerous pebbles both of 

 slate and quartz were found at a depth of six hundred feet m a 

 {ode, cemented by tinstone and sulphuret of copper. (Lyell, 

 student's Elements of Geology, p. 593). Not less instructive m 

 this connection are the observations of Mr. J. A. Phillips, on the 



