350 



SIK A. GEIKIE ON THE TERTIARY 



[May 1896, 



pieces of different lavas and rough slags are irregularly dispersed. 

 These stones occur here and there in rows, suggestive of more 

 vigorous discharges, the layers between the platforms of coarser 

 detritus being occupied by fine tuff. Some of the ejected blocks are 

 imbedded on end — an indication of the force with which they were 

 projected and fell nearly a mile from the crater. 



The upper parts of the tuff pass upward into fine yellow, brown, 

 and black clays a few feet in thickness, the darker layers being full 

 of carbonaceous streaks. On this horizon the coal of Portree was 

 formerly mined. The workings, however, have long been abandoned, 

 and, owing to the fall of large blocks from the basalt-cliff overhead, 

 the entrance to the mine is almost completely blocked up. One 

 wooden prop may still be seen keeping up the roof of the arch, 

 which is here a slaggy basalt. 



East and south-east of the Portree vent, extensive landslips of 

 the volcanic series and of the underlying Jurassic formations 

 make it hardly possible to trace the continuation of the tuff-zone 

 in that direction. To the south, however, at a distance of rather 

 more than 3 miles, what is probably the same stratigraphical 

 horizon may be conveniently examined from Ach na Hannait for 

 some way to the north of Tianavaig Bay. At the former locality 

 the calcareous sandstones of the Inferior Oolite are unconformably 

 covered by the section represented in fig. 12. At the bottom of 

 the volcanic series lies a sheet of 

 nodular dolerite with a slaggy upper 

 surface (a). Wrapping round the pro- 

 jections and filling up the depressions 

 of this lava comes a thin group of 

 sedimentary strata from 1 or 2 to 

 18 inches or more in thickness (&). 

 These deposits consist of hardened 

 shale charged with macerated remains 

 of linear leaves and other plant- 

 remains, including and passing into 

 streaks of coal, which may be looked 

 upon as probably occupying the same 

 horizon with the coal of Portree. But 

 here, instead of reposing on a mass of 

 stratified tuff, the carbonaceous layers 

 lie on one of the bedded lavas. The 

 tuff has died out in the intervening 

 3 miles, yet that some of the dis- 

 charges of volcanic detritus reached 



even to this distance, and that they took place during the accumu- 

 lation of these layers of mud and vegetation is shown by the 

 occurrence in the shales of pieces of finely amygdaloidal basalt 

 from less than 1 to 6 inches in length, likewise of lapilli of a 

 fine, minutely cellular, basic pumice, like some varieties of pala- 

 gonite. The overlying dolerite (c) becomes finely prismatic at its 



Fig. 12. — Section of the 

 volcanic series at Ach na 

 Hannait, south of Portree, 

 STcye. 



