GNEISSIC UOCKS. 47 



seems to depend entirely upon the presence or absence of a very minute 

 black licheU; which specially affects the highest^ most inaccessible, and 

 most weather-beaten parts of the rocky hills, but only when the surface 

 is much broken up by excessive jointing. The excessive development 

 of jointing probably depends upon some local difference in mineral 

 character not appreciable to the unassisted eye, but which doubtless 

 favors the growth of the black lichen. The dark coloring may be 

 seen* in some instances recurring at intervals in different parts of one 

 and the same bed of the granite-gneiss. A similar lichen occurs also on 

 some of the larger trap dykes, the summit ridges of which are excessively 

 broken up. The presence of trees, or the transit of much drainage water 

 over the surface, prevents the homogeneous covering of the surface by 

 the lichen in question, and hence its great development only under the 

 circumstances described. 



The schistose members of the gneissic series occupy, as already men- 

 tioned, a much smaller area in the country under 

 The schistose group. ..,-,, . . . 



description than do the massive granitoid varieties, 



but still they form very important features in various parts, and are 



deserving of much more study than the time at my disposal admitted of 



Five principal varieties must be noted among them ; 

 Five varieties. 



they are, 1*^, hornblendic schists ; 2nd, micaceous 



schists ; Srd, chloritic schists ; 4^^, hsematite schists ; and 5U, talcose 



schists. The order they are given in represents very fairly their relative 



importance as measured by the areas they occupy respectively. 



The most important and largest development, superficially, of the 



hornblendic rocks is that which occurs in the 

 Hornblendic schists. 



Maski band of schistose gneiss. By far the greater 



part of this band consists of hornblendic schists of different sorts, but 



with them, and apparently passing into them by slow and almost 



imperceptible gradation are various beds of chloritic and a few of 



talcose schist. Som,e beds are rather deficient in hornblende and very 



( 47 ) 



