Beds and Veins. 19 



kind must very often be irregular, especially as to their thick- 

 ness, and observation confirms this, inasmuch as it teaches us, 

 that such beds sometimes increase in their dimensions, some- 

 times become contracted, and sometimes are interrupted for 

 more or less considerable distances. The slate series of rocks 

 does not generally undergo thereby any change in the direction 

 of its structure, — at least any important change, and the change 

 it may present is only near these beds. If the increase of thick- 

 ness of such beds be considerable, and if an interruption im- 

 mediately ensue, the portions assume the form of irregular 

 masses, or of what are termed, lying irregular masses (Jtegen- 

 de stocke), and it not unfrequently happens, that the structure 

 of the slaty rocks abuts against these irregular masses : that is, 

 they are cut off by them. This happens sometimes by means 

 of a transition, somevv^hat in the manner above described ; some- 

 times, however, it happens in such a way, that the different 

 mountain masses are sharply united together, or are sepa- 

 rated from each by a distinct-concretion-surface. In the 

 first case, the rocky mass sends forth vein-like branches of 

 granular rock into the slaty mass, — such as are known more 

 especially in respect to granite, porphyry, greenstone, &;c., and 

 it is not unfrequent to find separate portions of the slate in 

 the mountain mass in contact with it, and likewise separate 

 portions of it in the slate. In the other, the phenomenon ac- 

 quires the aspect of a shift, and many examples of this kind 

 have been explained by shifts. But when we examine nar- 

 rowly the distinct concretion-surface, and sufficiently attend to 

 its nature in respect to its general aspect (viz. that it consists 

 of the distinct concretion-surfaces of the separate individual con- 

 cretions composing the rocks, and not of fracture or superficial 

 surfaces) ; when we attend likewise to its streaking (where such 

 exists), to its position, and to the bendings which it generally 

 makes, and which, in all the other hitherto mentioned relations, 

 are not compatible with the notion of a displacement of the rocks 

 in regard to each other ; we are easily convinced, that both 

 masses have been originally formed with the same relations, 

 and in the same position as at present,* and this conviction is of 

 great moment for the search after repositories of usefulminerals, 

 because all the other phenomena of superposition stand in ex- 



* Sec aate, p. 4. 



