W. H. Twenhofel — Silurian Section at Arisaig. 159 



base of the prominent rock mass known as Frenchman's Barn 

 appears a bed of red shale containing rounded bowlders overlain 

 by a thick-bedded series of dark-green stratified rocks. Both 

 the shale and the dark-green rocks have an almost vertical 

 attitude. Along lines of fracture all the rocks have a light- 

 green color and there is a conspicuous irregular band of like 

 color, varying from fifty 'to seventy-five feet wide, which is 

 said to have been traced for more than a mile (1886 : 9P). 

 Throughout the whole, at different points, are younger masses 

 of amygdaloidal trap. 



A chemical examination of the light-colored hornstone of 

 Arisaig Pier gave the following analysis, which was done in 

 triplicate to minimize possibilities of error : 



Si0 2 - 76-70 per cent. 



A1 2 G 3 12-98 " 



FeO 



FeOf °' 25 " 



CaO ._._ 0-54 



MgO 0-06 " 



K 2 4-14 



Na 2 0.._ 4-92 " 



H 2 0-59 



Ti0 2 trace 



C0 2 



P 2 5 0-31 



Total 100-53 " 



This analysis corresponds very closely to that of a rhyolite, 

 as is evident from the high per cent of alkalies present, which 

 would hardly be the case were the rock an altered sedimentary. 

 An analysis of the red shale shows that it is, in all probability, 

 of sedimentary origin. Examination of the rock in thin sec- 

 tions gave abundant evidence that the hornstones are none 

 other than rhyolites. The thin sections show that the hornstones 

 are made up of alternating bands of a very dense and less dense 

 material, in the latter Of which the minerals can be readily 

 determined. These consist of quartz and alkalic feldspar, the 

 quartz acting as a sort of sponge for the feldspar and giving 

 what is known as micropoikilitic structure. Numerous micro- 

 lites, of which the character has not yet been determined, are 

 present. Many small flakes of chlorite, along minute lines of 

 fracture and in places of alteration, explain, the green color 

 common in many places, and a section made from the green band 

 shows that the color in this particular section is also due to 

 chlorite. Pieces of angular fragments, characteristic of extrusive 

 volcanic rocks, are present in the dense bands. The thin sections 



