On Junctions of Bocks. 9 



ciated talc and chlorite slates, greywacke-slate, if there is suf- 

 ficient reason to separate it from clay-slate, and greywacke, 

 which is directly connected with the greywacke-slate. 

 Junctions of Bocks, — We generally find thegneiss next the gra- 

 nite ; but this is not without many exceptions, of which we have 

 remarkable examples in the Hartz, where greywacke-slate is in 

 direct contact with granite, and in Cornwall, where thekillas, a 

 rock intermediate between mica-slate and clay-slate, occupies 

 that position. Where gneiss and granite bound one another, 

 these rocks, which moreover are merely difi^erent in structure, 

 often pass directly into one another ; more rarely they are se- 

 parated from each other by true distinct-concretion-surfaces. 

 The stony structure of the gneiss generally changes in the 

 vicinity of the granite, and is bent for a short distance, be- 

 cause it as it were presses closely on the granite, whence ob« 

 servers have frequently deduced the existence of a mantle- 

 shaped arrangement of the beds ; and even the structm*e of the 

 granite itself not unfrequently undergoes a change, inasmuch 

 as it becomes sometimes more coarsely granular (as in what 

 are termed Stockscheidern), and sometimes more finely granu- 

 lar (as we find in the granite masses of Stockwerks in gneiss), 

 but generally it presents a tendency to the slaty structure. 

 However, there are likewise cases in which none of all these 

 phenomena exist, and where both rocks, with their prevailing 

 characters, are sharply joined and firmly grown together. 



The structure of the mountain-masses of gneiss, under which 

 is here understood what is generally termed stratification in 

 this and the other slaty rocks, is frequently cut by the granite, 

 or, as it is also expressed, the strata of gneiss abut against the 

 granite ; and this latter is far more correct, for cutting through 

 is merely a figurative idea, w^hieh includes more than can be 

 observed. Here, therefore, we have not the phenomenon of a 

 conformability , but that which has been termed unconformahi- 

 lity of the beds; but the consequences do not follow which 

 are generally drawn from a deviating disposition of beds :* 

 viz. that granite and gneiss are not of simultaneous origin, and 



* These consequences could only take place if the tabular structure of 

 gneiss and the other slaty rocks were real stratification. 



