part 2] THE SOUTH-WEST HIGHLANDS OY SCOTLAND. 87 



Nappes and their Travels. 



Definition o£ a nappe. — In 1909 a little party of Scottish 

 geologists was introduced to the wonders of the Pre-Alps under 

 the able guidance of Dr. L. W. Collet, now Professor at G-eneva, 

 I well remember a comment by Mr. H. B. Maufe on this occasion : 



' Alpine geolog-ists speak and think of nappes, while Scottish geologists have 

 been accustomed to concentrate their attention upon thrust-planes.' 



In the Southern Highlands, a combination of the two habits of 

 thought has proved advantageous. It is convenient to bestow 

 titles upon important structural masses, selecting for the honour a 

 nappe, a fold-core, or an entire fold, as the case may be ; it is also 

 convenient to give names in certain cases to major slides, both 

 thrusts and lags. 



On a previous occasion I have discussed the use of various 

 tectonic terms (1916, p. 25) ; but nappe was not included, since 

 I wished to present the main structural features of the district 

 considered without reference to such abstruse matters as original 

 order of succession and direction of movement. On the present 

 occasion I am venturing to express an opinion on these difficult 

 subjects, and accordingly offer a definition of the word nappe, as 

 follows: — A nappe is a mass brought forward to a notable 

 extent by recumbent anticlinal folding or by thrusting. 

 In deciding upon the basal limit to be assigned to any particular 

 nappe, one generally chooses some prominent thrust-plane ; failing 

 this, one is entitled to select the axial plane of some recumbent 

 anticline or syncline, according to local convenience. 



Direction of Movement defined and discussed. 



The definition of nappe given in the preceding section depends 

 upon a proper understanding of the words brought forward. 

 It may be of service to offer a few remarks in this connexion. 

 Often the rocks of a district, considered as a whole, show evidence 

 of marked horizontal compression at right angles to some parti- 

 cular line of strike ; while, considered in detail, they reveal a 

 differential horizontal movement of their upper layers as compared 

 with their lower. In such a case, movement is said to have 

 occurred in the direction in which the upper layers 

 have travelled relatively to the lower. At the present 

 time, this method of stating the facts is a well-rooted convention, 

 associated with a dependent terminology including such familiar 

 expressions as foreland and fore dee p. 



How line of movement is recognized. — A few words 

 may now be added regarding the criteria by which a geologist 

 judges of the direction of movement. In the first place, the Ime 

 of movement is at right angles to the strike of folds developed by 

 the movement, or is established even more immediately b}^ stria tion 

 of thrust-planes. In this connexion onlv one comment is necessary. 



