W. II. Twenhofel — Silurian Section at Arisaig. 147 



Manner and Place of Deposition. — The strata were formed 

 at no great distance from the shore, as is indicated by their 

 decidedly arenaceous character, the great amount of ripple 

 marking which is particularly characteristic of the upper levels, 

 and the lenticular character of the fine-grained sandstones that 

 are present almost throughout. These last show on a polished 

 or weathered surface wavy and cut-off lines of lamination indi- 

 cating rapidly changing wave action. 



Fossil Content. — The rocks, in general, are fossiliferous 

 throughout ; only one stratum, the " Red Stratum " of authors, 

 being without organic remains. They are particularly abund- 

 ant in the upper red shales and flags, and at many horizons in 

 the middle and lower shales. The impure limestones as a 

 whole contain few fossils, but a series of intercalated nearly 

 pure limestone lenses have them in more than ordinary abund- 

 ance. These lenses are of two classes : the one wide, but thin, 

 contains merely fragments ; the other, about three times as 

 wide as thick, contains many well-preseiwed fossils, usually of 

 but one species. 



Raised Beaches. — Three well-defined raised beaches stand 

 above the present one and slope gently to the northeast. 

 Their heights at Stonehouse brook, the top of the section, are 

 roughly estimated at twenty-five, seventy-five, and one hundred 

 and twenty-five feet. East of McAdam brook the lowest 

 merges into the present one. The lowest two still have very 

 steep cliffs, so their uplift must have been comparatively recent. 

 Whether these beaches rise in elevation westward is not known. 



Topography. — The faulting and fracturing of the rocks have 

 had little effect on the upland topography of the area underlain 

 by the Silurian. The brooks crossing the section flow in deep 

 and narrow gorges which do not owe their location to the 

 presence of fractured or faulted zones. Exceptions are 

 McDonald's and Arisaig brooks, which have chosen weakened 

 zones as points for breaking over the shore cliff. The gradi- 

 ent of all the streams is high, falls are present in each, and all 

 the evidence points to recent uplift of the region. 



In the shore line topography the story is a different one. 

 Weakened zones have made the smaller detail of the shore. 

 In the soft shales this is not so marked, but in the hard shales 

 and interbedded shales and limestones it is particularly evident. 

 The fractured zones erode readily and the cliff presents a 

 serrate appearance. 



The varying hardness of the rocks determines a higher order 

 of detail, shales zones being the location of coves, hard rocks 

 forming points. The latter is well shown by the hornstones 

 of Arisaig pier and by a double example in zone 35, where at 

 the present time is a prominent point ; and this appears to 



