Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/448

 salt-beds forming the side of the hill. Its interest arises from the evidence it affords 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 section there exposed. Rising over each other in successive beds, with

Fig. 10. — Natural Section at the Cliff of Bideann Boidheach, northwest end of the Scur of Eigg.

aa. Bedded dolerites and basalts, bb. Basalt dykes and veins, c. Ancient river-bed filled with conglomerate, p. Pitchstone of the Scur.

Hay Cunningham, in the paper already cited, states that the fossil wood really lies in the pitchstone itself ! Tbe 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 structure 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.