DUEING THE TEETIAEY PEEIOD IN THE BEITISH ISLES. 



83 



but less striking breccia occurs on the south coast of the same island, near Carsaig, made 

 up chiefly of pieces of quartzite and quartz.* 



c. Tuffs. — So far as I am aware, all the tuffs intercalated in the basalt-plateaux 

 consist essentially of basic materials, derived from the destruction of different varieties of 

 basalt-rocks, though also containing occasional fragments of older felsitic rocks, as well 

 as pieces of chalk, flint, quartz, and other non-volcanic materials. They are generally 

 dull, dirty-green in colour, but become red, lilac, brown, and yellow, according to the 

 amount and state of combination and oxidation of their ferruginous constituents. They 

 usually contain abundant fragments of amygdaloidal and other basalts. As a rule, they 

 are distinctly stratified, and occur in bands from a few inches to 50 feet or more in thick- 

 ness. The matrix being soft and much decomposed, these bands crumble- away under 

 the action of the weather, and contribute to the abruptness of the basalt-escarpments 

 that so often overlie them. 



Where the tuffs become fine-grained and free from embedded stones, they pass into 

 variously-coloured clays. Among these are the " beauxite " and " lithomarge " of Antrim. 





b da 



Fig. 19. — Breccia and Blocks of mica-schist, quartzite, &c, lying between bedded Basalts, Isle of Mull. 

 a,a, Bedded basalts ; b, Breccia ; d, Basic dyke. 



Associated with these deposits in the same district, is a pisolitic haematite, which has 

 been proved to occur over a considerable area on the same horizon. Many of the clays 

 are highly ferruginous. The red streaks that intervene between successive sheets of 

 basalt are of this nature (bole, plinthite, &c). The source of the iron-oxide is doubtless 

 to be traced to the decomposition of the basic lavas during the volcanic period. 



d. There occur also grey and black clays and shales, of ordinary sedimentary 

 materials, not infrequently containing leaves of terrestrial plants and remains of insects 

 (leaf-beds), sometimes associated with impure limestones, but more frequently with sand- 

 stones and indurated gravels or conglomerates containing pieces of fossil wood. These 

 intercalated bands undoubtedly indicate the action of running water and the accumulation 

 of sediment in hollows of the exposed flows of basalt at intervals during the piling up of 

 the successive lava-sheets that form the plateaux. 



The vegetable matter has in some places gathered into lenticular seams of lignite, and 

 * This is noticed by Mr Starkie Gardner, Quart. Jour. Geol. Soc, xliii. (1887) p. 283, note. 



