70 MR. A. GEIKIE ON THE AGE OP THE 



§ 4. Eelations of the Limestone to the othee Rocks or 

 THE District. 



Having now established the Silurian age of the older more 

 massive limestones of Strath, I may be permitted to add some 

 observations as to the structural relations of these rocks in the 

 general geology of the district, inasmuch as these relations reveal 

 some interesting facts in the geological history of the north-west of 

 Scotland. 



(1) The Cambrian or Torndon Sandstone. — Though the limestone 

 can be traced for more than five miles along the margin of the 

 red sandstone belt which rises in the heart of Strath, it is nowhere 

 seen resting directly on that rock. In the glen below Loch 

 Lonachan it forms the western side of the valley, while the 

 sandstone occupies the eastern side, the hollow itself coinciding 

 generally with the line of a fault which here separates the two 

 formations. Between this glen and Strath Suardal, as the main 

 valley of Strath is called, the boundary can be easily traced m a 

 singularly tortuous line over the northern portion of the ridge of 

 Beinn Suardal. Although, on the whole, the limestone forms an 

 anticlinal arch upon that ridge, yet its truncated and steeply 

 inclined strata, where they abut upon the red sandstone, show that 

 it does not lie in its normal position. I can only compare the 

 structure visible there with what is seen where portions of the 

 Silurian strata have been heaped up on the larger thrust-planes 

 in Sutherland. The non-occurrence of the " fucoid-beds " and the 

 quartzite here between the hmestone and red sandstone lead to 

 the inference that the lower portion of the Silurian series has been 

 cut out ; and the way in which the beds of the limestone have been 

 jumbled together inclines me to believe that the whole of the 

 limestone of Beinn Suardal is not in its original position, but has 

 been pushed westwards upon a floor of red sandstone. If this 

 inference is weU founded, it leads to the further conclusion that 

 probably the whole of the Lower Silurian limestone of Skye has 

 been similarly displaced. Along the flanks of the Eed HiUs we 

 find this rock broken up and intermingled with portions of the 

 quartzite. In some places these two rocks have been sheared 

 together so as to produce a calcareous quartz-schist. A good 

 example of this structure may be observed in Corry Uaigneich at 

 the foot of Blath Bheinn (Blaven). On the Sound of Scalpa also, 

 to the west of StroUamus, the red sandstone, quartzite, and 

 limestone have undergone enormous crushing. In Sleat the quartzite 

 and limestone have been extensively dislocated, while the red 

 sandstone along its south-eastern margin has been so sheared as to 

 pass into micaceous schist. Connecting these evidences of enormous 

 terrestrial disturbance with the lines of gigantic thrust which the 

 Geological Sui^ey has now traced from the north of Sutherland to 

 the shores of Lochs Keeshorn and Carron, opposite to Skye, I should 

 not be surprised to find that no portion of the older rocks in that 



