28 The Scottish Naturalist. 



"The Crystalline Rocks of the Scottish Highlands."— 

 Ever since the discovery of Silurian fossils in the rocks of 

 north-west Sutherland, it has been recognised that in that region 

 lies the key to the structure of the Scottish Highlands. Ac- 

 cordingly, when in the progress of the Geological Survey, the 

 mapping of the Highlands had to be undertaken, I determined 

 that a detailed survey of the Sutherland ground on the scale of 

 six inches to a mile should be made as a basis for the work. In 

 the summer of last year, a surveying party under the charge of 

 Mr. B. M. Peach, was stationed there with instructions to begin 

 by mapping the Durness basin. This duty was satisfactorily ac- 

 complished before the end of the season. The Silurian series of 

 Durness was ascertained to be about 2000 feet thick, and to con- 

 sist of numerous successive zones, which were traced on the six- 

 inch map, and discriminated in such a way as to be recognisable 

 should they be found to occur in the more complicated region of 

 the east. With this necessary ground-work well established, the 

 Eriboll tract was attacked this summer by Messrs. Peach and 

 Home. I had never myself had an opportunity of studying the 

 Eriboll section, which, from the days of Macculloch down to the 

 present time, have been such fruitful subjects of discussion. It 

 was a special instruction to the officers now intrusted with the 

 detailed survey of the region to divest themselves of any prepos- 

 sessions in favour of published views, and to map the actual facts 

 in entire disregard of theory. By the close of this last season the 

 structure of the Eriboll area had likewise been traced on the 

 six-inch maps, and I then went north to inspect the work. From 

 time to time during the summer, reports had been made to me of 

 the progress of the survey, but though from the published descrip- 

 tions of that tract, I was aware that its structure must be 

 singularly complicated, and although apprised of the conclusions 

 to which the surveyors, step by step, and almost against their will 

 had been driven, I was hardly prepared for the extraordinary ge- 

 ological structure which the ground itself presented, or for the 

 great change in the interpretation of the sections as given by Mur- 

 chison. 



No one cursorily visiting the ground, could form any notion of 

 its extraordinary complication, which could be satisfactorily un- 

 ravelled only by patient detailed mapping such as had never yet 

 been bestowed upon it. With every desire to follow the interpre- 



