W. H. Twenhofel — Silurian Section at Arisaig. 145 



washed on the north and northwest by the waters of the Straits 

 of Northumberland and separated from the Pre-Silurian rocks 

 to the southeast by a great fault. On the shore they are over- 

 lain unconformably at their western side by a series of con- 

 glomeratic sandstones of probable continental origin which are 

 thought to be of Carboniferous age (1886 : 89P), the two series 

 being separated by about fifty feet of fine-grained amygdaloidal 

 trap. To the southwest occur a series of continental sand- 

 stones and volcanic tuffs (1901 : 309) which are considered to 

 be of Devonian age. These form the Knoydart formation of 

 Ami (1901 b : 207 ; 1900 a : 303) and from them he has obtained 

 fish remains (1900 a : 309) and a series of tracks (1901 a : 330). 

 The Silurian is underlain unconformably (?) at Arisaig pier 

 and to the east by a series of banded hornstones, red conglom- 

 eratic shale, and syenites (1886 : 9P), which in many places have 

 been broken through or covered by fine-grained amygdaloidal 

 trap. A part of these rocks Honey man and Dawson considered 

 as altered Silurian (1864:339; 1891 : 565). Fletcher regards 

 them as possibly of Pre-Cambrian age, but states that little can 

 be said of them except that they are older than the " Medina." 

 It is probable that Fletcher is correct, as there appears to be 

 little evidence for the former view. 



Structure. — The entire section is well exposed on the cliff 

 and reef along the Strait and by several small brooks crossing 

 it from south to north ; two of them, Doctor's and Arisaig, 

 which are cut in the lower shales, crossing the entire section. 



The structure of the rocks is, according to Honeyman, that of 

 a synclinal fold (1864:333); but it can hardly be considered 

 as a typical syncline, a fact to which he also calls attention 

 (1864 : 335). It may be better characterized as a downfaulted 

 block which has broken south of its mid line, giving rise to a 

 trough which plunges to the southwest. 



This conclusion is derived from the fact that the lowest beds 

 of the Silurian have not been seen on the southern side of the 

 area, and by the evidence for faulting given by the topography. 

 The Silurian hills are separated from Eigg mountain or Maple 

 ridge, the Cambro-Silurian upland, by a low area called " The 

 Marshes," which extends approximately parallel to the shore 

 for ten or more miles and marks the fault zone, as Fletcher has 

 pointed out. The beds are tilted away from this fault zone for 

 a short distance, but this is what should be expected. The 

 trough thus formed is much modified by secondary folds and 

 faults which in the lower part of the section reach a maximum. 

 Hence the rocks range from a horizontal attitude just west of 

 the mouth of Arisaig brook to a vertical and overturned atti- 

 tude at Beach Hill Cove. The general dip of the rocks of the 

 shore is toward the southwest. 



