DURING THE TEETIAEY PERIOD IN THE BRITISH ISLES. 81 



inequalities in the accumulation of the lavas. If the next series of flows came from 

 another vent and accumulated against such a gentle slope, it would be marked by a slight 

 unconformability. Structures of this kind are much rarer than we should expect them 

 to be, considering the great extent to which the plateaux have been dissected and laid 

 open in cliff sections. Near the west end of Glen More, in Mull, I observed a hillside 

 where, as seen from a little distance, one series of basalts appears to be banked up against 

 the edges of another. 



A common feature in all the plateaux is the intervention of a red layer between 

 successive sheets of basalt. These red streaks form a striking feature on many sea-cliffs, 

 and emphasise the bedded character of the volcanic series. Examined more closely, the 

 thin red line is found to be a layer of clay or bole which shades into the decomposed top 

 of the bed whereon it lies, and is usually somewhat sharply marked off from that which 

 covers it. This layer has long, and I think correctly, been regarded as due to the 

 atmospheric disintegration of the surface of the basalt on which it occurs, before the 

 eruption of the overlying flow. It varies in thickness from a mere line up to a foot or 

 more, and it passes into the tuffs and clays which are sometimes interposed between the 

 sheets of basalt. 



2. Fragmented Rocks. — While the plateaux are built up mainly of successive flows 

 of basaltic lavas, they include various intercalations of fragmental materials, which, though 

 of trifling thickness, are of great interest and importance in regard to the light which 

 they cast on the history of the different regions during the volcanic period. I shall 

 enumerate the chief varieties of these rocks here, and give fuller details regarding their 

 stratigraphical relations and mode of occurrence in connection with the succession of beds 

 in each of the plateaux. 



a. Volcanic Agglomerates. — Under this name are included all the tumultuous 

 unstratified masses of fragmentary materials which fill eruptive vents in and around the 

 plateaux. The stones vary in size up to blocks several feet in diameter. They consist 

 for the most part of basalts, often highly slaggy and scoriaceous, also fragments of 

 different acid eruptive rocks (generally felsitic in texture), with pieces of the non-volcanic 

 rocks through which the volcanic pipes have been drilled. The paste is granular, 

 dirty-green or brown in colour, and seems generally to consist chiefly of comminuted 

 basalt. 



b. Volcanic Conglomerates and Breccias in beds intercalated between the flows of 

 Basalt. — These are of at least three kinds, (a) Basalt-conglomerates, composed mainly 

 of rounded and subangular blocks of basalt (or allied basic lava), sometimes a yard or 

 more in diameter, not unfrequently in the form of pieces of rough slag or even of true 

 bombs, embedded in a granular matrix of comminuted basalt-debris. In some cases, the 

 stones form by far the most abundant constituents of the rock, which then resembles 

 some of the coarse agglomerates just described. On the east side of Mull, for example, 

 the slaggy basalts of Beinn Chreagach Mhor are occasionally separated by materials of 

 this character. But such intercalations are seldom more than a few feet or yards in 



