﻿1861.] 



HARKNESS HIGHLANDS AND N. IRELAND. 



257 



in the Highlands ; and these rocks, generally designated gneiss, 

 mica-slate, chlorite-slate, and clay-slate, have been looked upon as 

 possessing a distinct arrangement as to superposition to each other. 

 From what I have seen in the several areas which have been under 

 my examination, it appears to me that these rocks, with names ex- 

 pressing lithological nature, have no definite relation as concerns 

 geological position, although in some small areas this seems to be 

 the case. Looking at these metamorphic rocks with -reference to 

 general results, I have arrived at the conclusion that, in whatever 

 mineral condition they present themselves, this mineral condition 

 is a purely local character, and that, in the same geological zone, 

 in one spot we have clay-slate, in another chlorite-slate, in a 

 third mica-slate, and in a fourth gneiss. These several rocks are 

 altered deposits of varying shales and sandstones which were de- 

 posited during the same period in different portions of the older 

 Silurian seas. These metamorphic rocks, to which I apply the 

 general term " gneissose," although the word is in many instances a 

 bad one, are spread over too great an extent in the geological maps 

 of Scotland ; and they are frequently represented as occupying large 

 areas, over which granite or some other form of plutonic rock 

 really prevails. And, although rocks of the latter nature seem to 

 be so amply developed in the Highlands, great injustice is in many 

 instances done to ix>cks of this description in the geological maps of 

 Scotland. 



With reference to the mode of arrangement of the other strata 

 which are associated with the gneissose rocks (viz. the quartz-rocks 

 and limestones), it has generally been assumed that these latter 

 occupy a higher position than the former. This inference I have 

 reason to conclude is not the result of observation, but has ori- 

 ginated from the Wernerian theory and classification of rocks, — the 

 more crystalline gneissose rocks having been looked upon as primi- 

 tive, while the less crystalline and more distinctly stratified quai'tz- 

 rocks and limestones were regarded as transition. This idea appears 

 to have prevailed in the mind of Macculloch ; and this order, in the 

 arrangement of the stratified masses of the Highlands propounded 

 by him, has been to a great extent adopted by geologists without 

 inquiring into the basis on which it rests. 



The principles which actuated Macculloch in his classification of 

 the Highland rocks may be ascertained by referring to his memoir 

 " on Quartz-rock," where, after quoting Playfair with reference to 

 the structure of Schihallien, he goes on to say, " If this ridge is 

 connected by any system of alternation with the sandstone of Glen 

 Lyon, mica-slate will appear to be a rock formed posteriorly to, or 

 alternately with, a rock of recomposed structure. Thus a primitive 

 will be found to alternate with a transition one, — an anomaly which 

 either renders this distinction as useless as it is artificial, or compels 

 us to modify the definition of transition rocks or to form that total 

 change of arrangement which I have more than once suggested with 

 regard to primitive and transition classes*." 



* Geol. Trans., vol. ii. p. 470. 



T 2. 



