10 M. Mohs's Summary of Geopiostical Phenomena. 



even that they are dissimilar in nature ; for the forms * in 

 which the former appears in the latter, and the above-men- 

 tioned transitions, incontrovertibly prove the contrary. 



In the vicinity of the mutual boundary sometimes the one, 

 sometimes the other, sometimes both of the above-mentioned 

 mountain-rocks, contain minerals which do not belong to their 

 component pai'ts, such as garnet, tourmaline, and hornblende ; 

 the last more especially, when the granite is syenitic, or is ac- 

 tually a syenite, a rock which is almost too closely connected 

 with granite to entitle it to be distinguished by a separate name. 

 Connected with this occurrence, but of more importance for 

 the search after usefiU minerals, is the fact that, at the junc- 

 tion of the rocks, repositories of these minerals not unfre- 

 quently occur, which do not belong to the composition of the 

 general mass. Such junctions, therefore, must be carefully 

 attended to, and where there is an opportunity, must be mi- 

 nutely examined. 



When mica-slate is in immediate contact with granite, the 

 phenomena are essentially almost the same as we have de- 

 scribed above. Only the granite is not unfrequently very 

 rich in quartz, and the mica-slate often acquires felspar near 

 the junction, and is thus converted into gneiss. Clay-slate 

 often exhibits a greater diversity of aspects near the granite. 

 It sometimes assumes the characters of gneiss or hornblende ; 

 and what is called greywacke-slate becomes converted into a 

 rock which is called hornfels in the Hartz, which passes di- 

 rectly into the greywacke-slate, and is yet connected with the 

 granite, inasmuch as it contains some felspar. This rock, 

 moreover, occurs at a greater distance from the granite in 

 greywacke rocks, and hence is not connected with the imme- 

 diate vicinity of greywacke rocks to granite. 



No determinate arrangement of the Frimitive and Transition 

 Bocks. — The individual mountain-rocks of the slate series usual- 

 ly alternate with one another in the form of beds ; but there is 

 no fixed sequence in this alternation. Sometimes the mica- 

 slate lies on the gneiss, then the clay -slate on the mica-slate, 



* Some of these forms arc among the characters by which contempora- 

 neous formation is^dircctly recognised. 



