114 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



Analysis of the Rock of Traprain Law. 



SiOa. 



Al,03. 



Fe.^03, PeO, MnO. 



CaO. 



MgO. 



K2O, NaaO. 



Total. 



56-36 



25-00 



10-62 



3-83 



1-35 



4-01 



101-17 



(f) Tuffs. — The fragmentary ejections of the Plateaux vary in 

 texture from the finest-grained tuffs to coarse agglomerates.^ As 

 they have been derived from the explosions of porphyrite-lavas, they 

 consist mainly of porphy rite debris. They are often deep red in colour, 

 as for example those of Dunbar, but are most frequently greenish. 

 They have a granular texture, due to the small lapilli of various 

 porphyrites embedded in a fine dust of the same material. Grains 

 of quartz may frequently be detected even in the finer tuffs. Some 

 of these may have been ejected from the volcanic vents or may have 

 been grains of sand in the ordinary sediment of the sea-bottom. 

 Both at the base and at the top of the plateau-series, the tuffs are 

 interstratified with and blend into sandstones and shales, so that 

 specimens maybe collected showing a gradual passage from volcanic 

 into non-volcanic detritus. In many of the tuffs of the necks 

 fragments of sandstone and other stratified rocks occur, representing 

 the strata through which the vents were drilled. In the tuffs of the 

 Eaglesham district pieces of grey and pink granite have been met 

 with which, if they are portions of an old granite mass below, must 

 have come from a great depth. ^ In the coarser tuffs and agglo- 

 merates a larger variety of lava-form rocks is to be found than can be 

 seen among the bedded lavas of the Plateaux. They include felsites 

 and quartz-porphyries, and more rarely basic lavas (diabases, &c.). 



III. Geological Steuctuee. — The structure of the various pla- 

 teaux presents a general similarity, with many local variations. 

 Each plateau is built up entirely or almost entirely of sheets of 

 volcanic material, the intercalations of ordinary sedimentary layers 

 being few and unimportant and usually occurring either towards 

 the base or the top of the volcanic series. The vents of eruption 

 are in some instances still to be recognized on the plateaux them- 

 selves. More usually they occur on the lower ground flanking the 

 volcanic escarpments, where they have been laid bare by denudation. 



" ^ For accounts of these rocks, see Explanation of Sheet 33, Geol. Surv. 

 Scotland, p. 32; Sheet 22, pp. 11-14 ; Sheet 31, pp. 14-17. 

 2 Explanation of Sheet 22, Geol. Surv. Scotland, p. 12. 



