Rev. T. G. Bonney — Cirque in the Sills of Skye. 535 



directed my attention to the subject ; and also to the Literary and 

 Philosophical Society of St. Andrews, for the free permission which 

 they accorded to me to examine and make notes of the beautiful speci- 

 mens from Dura Den, in the Geological collection of their Museum. 



EZPLANATION OF PLATE XIV. 



Fig. 1. Phaneropleiiron elegans, natural size, from the Lower Carboniferous 

 Limestone of Burdiehouse. 



Fig. 2. Another specimen, natural size, -without the head, but showing the 

 extreme tip of the tail. The dorso-caudal flu is injured in front, and the direction of 

 the caudal axis slightly distorted, so as to spoil the beautiful, though gentle, upward 

 curvature which in the other specimens it is seen to have towards its termination. 



II. — On a Cikque in the Syenite Hills of Skye. 



By T. G. Bonney, M.A., F.G.S. 

 (Eead before the Cambridge Philosophical Society, October 30, 187L) 



IN a communication to the Geological Society,^ " On the formation 

 of Cirques, and their bearing upon theories attributing the 

 excavation of Alpine valleys mainly to the action of Glaciers," 

 I confined myself to the examination of Cirques in sedimentary 

 strata, and these to a large extent calcareous. I observed this 

 restriction because the most typical examples, so far as I could 

 ascertain, lay in non-crystalline districts, and because my own 

 experience had not presented to me any very striking instances in 

 other districts. The present brief paper is an attempt to show that, 

 under favourable circumstances, the principles there laid down may 

 also be applied to regions wherein crystalline rock exists. 



A Cirque may be defined as a variety of what in Scotland is called 

 "a corrie," viz., a more or less semicircular recess in the heart of a 

 mountain region, and at or near the head of a valley, inclosed by 

 a high wall of approximately vertical cliffs. 



In the above-named paper I attempted to show that these Cirques 

 could not be explained by : (a) what earlier geologists termed a 

 crater of elevation ; (/3) or by any kind of marine action, such as 

 has excavated some of the coves which may be seen among coast 

 cliffs ; (7) or by the action of a glacier ploughing up the ground ; 

 and that the only explanation, consistent with the phenomena which 

 they presented, was to attribute them to the erosive action of several 

 small streams of water, derived from sources higher up, which, 

 aided by atmospheric denudation, — rain, heat, frost, — cut their way 

 gradually backward into the mountain side. 



At that time I put aside the question of how far a favourable 

 configuration of the surface was needed before these agents could 

 act, and by what physical causes it had been produced ; all I 

 attempted to establish was, that Cirques in their present condition 

 are the result of the joint action of running water and meteoric 

 agents; and the same limitation must be supposed to hold in the 

 present paper. 



1 Quart. Journ. Geol. Soc, vol. xxvii. p. 312. 



