GENERAL COMPARISON OF 



and repeated alternation of every member; but the mi- 

 caceous schist is generally next to the quartz rock, and 

 the thin chlorite schist accompanies the thick and com- 

 pound variety. Thus the contrast between the beds 

 of different dimensions, no less than the perpetual in- 

 terchange of colour and the extreme regularity of the 

 strata, render the whole very conspicuous, and strongly 

 distinguished from the shapeless and contorted beds of 

 the series of micaceous schist. 



It is now necessary to enumerate the subordinate 

 varieties which are found in this series ; nearly the 

 whole of which may be observed in Gigha, in the description 

 of which they were purposely omitted for the sake of 

 bringing the whole into one condensed view. The want 

 of distinctive terms renders an enumeration of the in- 

 gredients and of their mode of disposition necessary. 



The hornblende schist is found under the following 

 different modifications : 



1. Simple schistose hornblende, either scaly or fibrous. 



2. A mixture of hornblende and felspar ; the former 

 scaly, the latter arenaceous, and the structure of the rock 

 schistose. 



3. An alternating mixture of hornblende and felspar, 

 both laminar ; the laminae very decided^ and the rock 

 easily splitting into thin slates. 



4. A mixture of hornblende and felspar, so minute as 

 to present an uniform grey aspect. The substances are 

 scarcely distinguishable by the eye, and the rock is 

 schistose. 



5. A mixture of felspar and hornblende, not schistose 

 in the fracture, but regularly alternating with all the 

 schistose rocks.* 



* This rock might, in hand specimens, be considered a member of the 

 trap family. It is ranked here on account of its obvious and intimate 

 affinity with these strata, not because we are at liberty to supersede the 

 use of a mineralogical term when the substance to be described does not 

 agree in its geological relations with those to which that is generally 



