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PE0CEEDING8 OP THE GEOLOGICAL SOCIETY, 



[Apr. 5, 



salt-beds forming the side of the hill. Its interest arises from the 

 evidence it aifords of the prolongation of the old valley eastward, 

 and consequently of the former extension of the precipice of the 

 Scur considerably beyond its present front. 



It is at the extreme north-western extremity of the pitchstone 

 ridge, however, that the most remarkable exposure of the ancient 

 river-bottom is now to be seen. Sweeping along the crest of the 

 plateau the ridge reaches the edge of the great precipice of Bideann 

 Boidheach, by which its end is truncated, so as to lay open a sec- 

 tion of the gravelly river-bed along which the pitchstone flowed. 



The accompanying diagram (fig. 10) represents the natural sec- 

 tion there exposed. Rising over each other in successive beds, with 



Fig. 10. — Natural Section at the Oliff of Bideann Boidheach, north- 

 west end of the Scur of Eigg. 



a a. Bedded dolerites and basalts, bb. Basalt dykes and veins, c. Ancient 

 river-bed filled with conglomerate, p. Pitchstone of the Seiir. 



Hay Cunningham, in the paper already cited, states that the fossil wood really lies 

 in the pitchstone itself ! The actual position of the wood, however, in the breccia 

 and conglomerates underlying the pitchstone is beyond all dispute. I have my- 

 self dug it out of the bed. The geological horizon assigned to this conifer, on 

 account of its supposed occurrence among Oolitic rocks, being founded on error, 

 no greater weight can be attached to the identification of the plant with an 

 Oolitic species. Our knowledge of the specific varieties of the microscopic struc- 

 ture of ancient vegetation is hardly precise enough to warrant us in definitely 

 fixing the horizon of a plant merely from the examination of the minute texture 

 of a fragment of wood. From the internal organization of the Eigg pine, there 

 is no evidence that the fossil is of Oolitic age. From the position of the wood 

 above the dolerites and underneath the pitchstone of the Scur it is absolutely 

 certain that the plant is not of Oolitic but of Tertiary date. 



