4 M. Mohs's Summary of Geognostical Phenomena, 



as such surfaces (as we are taught by the consideration of com- 

 pound minerals, twin crystals, &c.)> can only be produced when 

 the individuals or component parts of the one compound mass 

 come in contact with those of the other, in and during their 

 original formation, so it follows that these angular pieces of 

 granite, throughout its whole mass, and with them the surfaces 

 pointed out, have not only been produced at the same time, but 

 also, that the bounding surfaces or rents, as they have been 

 termed, are not true rents (zerspaltungen), but surfaces of dis- 

 tinct concretions, of a higher order, however, than those which 

 bound the individual concretions of which a rock is composed ; 

 and that these angular pieces are actually distinct concretions. 

 Compound mountain-masses likewise, that is, those consisting 

 of dissimilar substances, such as granite and limestone, por- 

 phyry and gneiss, basalt and chalk, &c., are often bounded by 

 distinct-concretion-surfaces, whichbelong to a still higher order. 

 From these considerations the following deduction has been 

 drawn, and it is one which is of great consequence for the 

 whole of geognosy : that when two similar or dissimilar moun- 

 tain-masses, or any other kind of simple or compound mineral 

 substances, come into contact with one another, in such a man- 

 ner, that they are only separated from one another by surfaces, 

 regarding which it can be shewn that they possess the charac- 

 ters of surfaces of distinct concretions, these rocks or minerals 

 must be of contemporaneous origin ;* and this deduction, con- 

 taining one of the most important criteria of contemporaneity 

 of formation, is of the most extensive utility, more particularly 

 for the correct determination of the relations of the beds of 

 rocks, an acquaintance with which is the only guide that we 

 can follow, without fear of getting into a wrong path, when we 

 wish to investigate mountains, or to search in them for useful 

 minerals. 



• We do not understand by contemporaneous origin, an origin at a fixed time, 

 but one during an uninterrupted time, in which no pause occurred in the for- 

 mation, of whatever longer or shorter duration it may have been, and which 

 admits of farther determination. Hence, two rocky masses can have been 

 produced at once, or the one can have been formed at an earlier or later pe- 

 riod than the other ; if their periods of formation are not separated by a 

 sensible interval, they are considered by us as contemporaneous. 



