12 M. Mohs's Summary of Geognostical Phenomena, 



knowledge by direct observation, of the phenomena of a par- 

 ticular mountain-district or a particular portion of it, without 

 allowing ourselves to be guided by any theory or by any sys- 

 tem ; and this knowledge is, as the sequel will shew, of the 

 utmost importance. 



Transitions of JRocks. — The rocks composing the individual 

 members of the slate series pass immediately into one another. 

 The transitions of the gneiss into mica-slate, and of the mica- slate 

 into clay-slate (including grey wacke-slate) are generally known. 

 But these rocks likewise pass into the common or granular grey- 

 wacke, and this transition deserves to be more attentively con- 

 sidered. The gneiss first of all has its structure altered, and 

 becomes granular, without acquiring any resemblance to gra- 

 nite (for there are other varieties of gneiss which form the 

 passage into granite), the mica loses its usual characters, more 

 especially the perfection of its cleavage, and begins to resem- 

 ble clay-slate, and the felspar and quartz appear of a grey 

 colour passing into green, which is generally the colour of the 

 whole mass. The structure is at first angulo-granular, but 

 not without occasionally passing back into the slaty ; and the rock, 

 when the size of the concretions is diminished, bears so great a 

 resemblance to some sandstones that we are hardly able to discri- 

 minate between them. On the other hand, the concretions or 

 grains lose less in size than in the sharpness of their edges; the 

 mica becomes closely united to them, and as its quantity and that 

 of the felspar gradually diminish, rocks are formed, which it 

 would not appear strange to us to meet with in the most cha- 

 racteristic greywacke district. This transition, which occurs 

 very frequently under all modifications, only admits of a gene- 

 ral account here ; and we need not further explain in what 

 consists the application of its knowledge in aid of the search 

 for useful minerals. 



But, nevertheless, another circumstance, connected with 

 these transitions, deserves some observations. It is generally 

 the case, that the mountain-masses pass into one another just 

 as they lie on and over one another ; and in such a manner, 

 that when on a bed of gneiss there reposes a bed of mica-slate, 

 and on the latter a bed of clay-slate, the first passes into the 

 second, and the second into the third. We may term this the 



