52 ME. A. HABKEE ON THE GEOLOGICAL [Feb. T906, 



but probably also later tban the basic dykes ; for one at least of 

 these stops abruptly on encountering the felsite, as if cut off by it. 

 £0 far as the evidence goes, therefore, this Grulin felsite may well 

 be of the same age as the pitch stone. The felsitic rocks near Laig 

 Farm and at the extreme north of the island are earlier, being 

 intersected by the basic dykes ; and they are thoroughly acid rocks, 

 while the Grulin felsite is essentially felspathic. Otherwise unique 

 in the district, this rock has petrographical characters in common 

 with the felsite-sheets of the Sgurr, although, as is to be expected, 

 it is not of such fine texture. A partial analysis by Dr. Pollard 

 enables us to compare its composition with that of the pitch- 

 stone : — 



-D-, 7 , Grid a' 



felsite. 



SiO., 63-3-1 '58-13 



Na„b 4-56 4-57 



K 2 4-50 4-85 



The resemblance in respect of the alkalies is very close. 



III. Feagmextal Accumulations and Fossil Wood. 



In two places accumulations of fragmental materials are seen 

 immediately beneath the pitchstone, or beneath the pale decomposed 

 band which has been included as a component part of the pitch- 

 stone-complex. These fragmental accumulations are of importance, 

 as constituting a part of the evidence upon which Sir Archibald 

 Geikie has based his interpretation of the structure of the Sgurr of 

 Eigg. He considered them to be of the nature of river-gravels, laid 

 down on the floor of a va]ley at this place, and subsequently covered 

 by a pitchstone lava-stream. The two localities present different 

 features, and will be described separately. 



"We take first the section at Bidein Boidheach, where the ridge, 

 running north-westward, is abruptly cut off by the sea-cliffs. Sir 

 Archibald Geikie has given a figure 1 of this place, and by the 

 kindness of Mr. A. S. Beid, F.G.S., I am enabled to illustrate it by a 

 photograph, taken by him, under considerable difficulties, from the 

 sea (PI. IV and fig. 4, p. 53). The lower part of the section shows 

 the usual alternations of basalt-lavas and dolerite- sills lying nearly 

 horizontally. At the top is the pitchstone-sheet, its base at this 

 place dipping inward on both sides of the ridge in a way which, 

 taken alone, might seem to accord well with the supposition that 

 the pitchstone occupies the bed of a valley. Below this, and filling 

 a hollow in the basalt-group, is the fragmental accumulation to be 

 noticed. As seen in the cliff-section, it has a maximum thickness 

 of perhaps 100 feet, rapidly thinning away in both directions. It 

 might represent either a cross-section of a deposit extending along 

 the direction of the ridge, or a small funnel-shaped mass which 

 happens to be intersected by the present line of the cliff. I interpret 



1 Quart. Journ. Geol. Soc. toI. xxvii (1871) p. 308. 



