534 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 25, 



justified by their actual height ; for only two points reach an eleva- 

 tion of 1800 feet. They exhibit for the most part a rounded and 

 flowing outline, but in many places are broken and rugged, and 

 show considerable slopes, the most abrupt perhaps being Suainabhal 

 in the north-west. 



The geologist is not long in Lewis without being struck with the 

 enormous number of fresh-water lakes that almost everywhere 

 diversify the face of the island. These are of all sizes, from mere 

 ponds up to lakes two, three, and even six miles in length. Taken 

 altogether they must occupy no inconsiderable portion of the surface 

 of the island. One may count upwards of fifty in an area of only 

 four square miles. They are most abundantly developed upon the 

 low, undulating, and sometimes rocky ground that extends from 

 shore to shore along the base of the southern mountain-tract. North 

 of a line drawn from Loch Carloway to Stornoway, they become less 

 numerous, although they are still sufficiently plentiful to form a 

 striking feature in the scenery. In the southern mountain-tract 

 there is hardly a valley that does not contain one or more. 



III. Geological structure of Lewis. 



The greater portion of Lewis, as Macculloch long ago pointed out, 

 consists of gneiss and its varieties. The only other rocks met with 

 in the island are granite (at Delbeag), and red sandstone and con- 

 glomerate of Cambrian age, which cover a portion of the Eye 

 peninsula and the shores of Stornoway Harbour at Arnish Point. 

 The same deposits are continued north as far as Gress. 



The Lewisian gneiss (Murchison's Fundamental or Laurentian 

 gneiss) is generally coarse-grained and of a greyish pink colour. 

 Occasionally, however, it assumes a finer texture and shows darker 

 tints : and here and there it seems to pass into a kind of argillaceous 

 schist and to contain interbeddings of several crystalline rocks, 

 which, as far as my own observations went, appear to be merely 

 varieties of the gneiss itself. In these rocks sometimes felspar, 

 sometimes quartz forms the prevailing mineral ; and with these 

 are associated now mica and again hornblende. All the gneissic 

 rocks in the island weather more or less rapidly. Below certain 

 glacial deposits, I found them soft and friable to a depth of several 

 feet ; and even projecting knobs and bosses of harder rocks often 

 showed on freshly fractured faces that weathering had likewise 

 affected them for several inches from the surface. 



As a rule the stratification of the gneissic rocks is well-marked, 

 the beds having a prevalent north-east and south-west strike, 

 with a south-east dip, generally at a high angle. In some places, 

 however, the direction of dip changes to different points of the 

 compass, and the bedding not unfrequently becomes crumpled, 

 contorted, and obscure. 



I mention these facts concerning the geological structure of 

 Lewis merely to make clear the remarks which follow. My time 

 spent in the island was fully occupied in studying the glacial 



