E M. Mohs's Summary of Geognostical Phenomena. 



means of a third unity of the same degree of extent, there can 

 exist between these three unities no sharp boundaries ; and 

 nature confirms the idea in the case of the different classes of 

 mountain-masses, that there are really no sharp boundaries 

 between them. The division, therefore, has no sufficient foun- 

 dation, and therefore no value. It is necessary, in the search- 

 for useful minerals, to have this well in view, in order that we 

 may not lay too much weight on the division of mountain- 

 masses, and especially on the classes of the primitive and tran- 

 sition rocks, inasmuch as we might thus be restrained from 

 making a search for any thing in a mountain-mass of one 

 division, regarding which he believes that it is only to be 

 found in one belonging to the other. 



On Formations. — With respect to the subdivision of the 

 classes, or of the mountain-masses generally, into forma- 

 tions, whose limits are chiefly founded on a certain sequence 

 of superposition, it is not much better, especially in the pri- 

 mitive and transitive classes, for this sequence (which is also 

 termed the series in regard to relative antiquity of the 

 mountain-masses), in respect to the masses which are dis- 

 tinguished, is not only not constant^ and on the contrary 

 subjected to a diversified change, but the mountain-masses 

 of these different formations pass into one another ; as all 

 geognosts know and admit, without sufficiently attending 

 to the consequences. It is always dangerous, and quite con- 

 trary to a scientific mode of proceeding, to employ a notion 

 of whose correctness and precision we are not convinced ; 

 and it is so much the more unsafe to do this in an occupa- 

 tion which is so difficult, and which requires so much at- 

 tention, as the searching after useful minerals, especially 

 when we previously know the untenable nature of such a no- 

 tion. Nevertheless, the idea of formations, although it can- 

 not succeed in determining the individual formations with the 

 requisite certainty and sharpness, is not only very useful for 

 those who devote themselves to this occupation, but also ver}^ 

 important as a valuable guide ; and I shall, in another place, 

 speak at greater length of this utility and importance. 



The mountain-masses which constitute the slate rocks are 

 more particularly gneiss, mica-slate, clay-slate, with its asso- 



