']G STKATIGKAPIIICAL GEOLOGY. 



Veins and Dykes. 



Almost every one has noticed, where eruptive rocks come in con- 

 tact with those that are stratified, near the hne of contact, they often 

 send off veins into the surrounding strata. These veins do not differ 

 from the general mass, except that they are generally finer grained. Dr. 

 Hunt describes what he calls granitic veinstones, and he regards them as 

 concretionary. These are not, however, on the borders of great areas of 

 eruptive rocks. "They are," he says, "to the gneisses and mica schists, 

 in which they are generally inclosed, what calcite veins are to stratified 

 limestone; and although long known, and objects of interest from their 

 mineral contents, have generally been confounded with intrusive gran- 

 ites." The theory as to the way in which veins have been formed has 

 been a matter of controversy from the time of Agricola, who, in 1546, 

 was the first to propound a theory of veins. He supposed that the prin- 

 cipal agents were water, which dissolved the enclosed rock, heat, and 

 cold. Werner was the first to fully develop the aqueous theory. The 

 fissures he supposed to be caused by contraction, in consequence of dry- 

 ing and various other causes ; and the vein to be formed from a wet and 

 mostly chemical solution, which covered the region where the fissures 

 existed. Lasius supposed that the material was introduced in a state 

 of aqueous solution, as mineral water; Lehmann, that the material was 

 brought into the fissures by ascending steam ; Becher, that the matter 

 was introduced in a gaseous condition by sublimation ; Fournet, that the 

 material has been introduced by an igneous and purely anhydrous fluid 

 injection, and solidified in the fissures. Scheerer conceives the congeal- 

 ing granite mass to have been impregnated with a highly-heated aqueous 

 solution, "which, under great pressure, oozed out, penetrating even the 

 stratified rocks in contact with the granite, filling cavities and fissures in 

 the latter, and depositing therein crystals of quartz and of hornblende, 

 the arrangement of which shows them to have been of successive 

 growth." 



According to Elie dc Ikaumont, the coarsely crystalline granitic veins 

 were injected. He supposed the material to be derived from the con- 

 gealing granitic mass. The veins which are characterized by a symmet- 

 rical banded structure he regards as concretionary. Von Cotta is in 



