CHAP. XXVI 
MATERIALS ERUPTED BY THE PUYS 
423 
to be distinguished by tlieir conspicuou.s stratification, and especially by their 
rapid alternations of coarser and finer material, by the intercalation of shales, 
limestones, sandstones or ironstones in them, and by the insensible grada- 
tions by which they pass both vertically and laterally into ordinary sedi- 
ments. Occasional large blocks or bombs, indicating some paroxysm of 
explosion, may be observed even among the finer tuffs, shales and other 
strata, which round the sides of these masses have had their layers bent 
down by the fall of heavy blocks.'^ Many of the bedded tuffs contain 
fossils, such as crinoids, corals, brachiopods, fish-teeth or macerated fragments 
of land-plants. Coal-seams also are occasionally interstratified among them. 
Of the finer kinds, the best example is furnished by a remarkable group 
of “ green and red marls ” which lie above a seam of coal (Houston Coal) 
in the Calciferous Sandstones of West Lothian.^ These strata, which differ 
much from any of the rocks with which they are associated, are exceedingly 
fine in grain, dull sage-gi'een and brownish or chocolate-red in colour, not 
well laminated like the shales, but breaking under the influence of weathering 
into angular fragments, sometimes with a conchoidal fracture. They look 
like indurated mud. Mr. H. M. Cadell, who has recently re-examined 
them in connection with a revision of the Geological Survey Map (Sheet 32) 
has found them passing into ordinary granular tufl‘. 
I’alagonitic-tuff is of frequent occurrence. It is met with in the Firth 
of Forth district,® and Mr. Watts has detected fragments of palagonite among 
the tuffs of the Limerick basin. 
> AnU, p. 36, and Figs. 1,’i and 151. See also Geol. Mag. i. (1864), p. 22 ; Tram. Roy. Soc. 
Edin. vol. xxix. (1879) p. 515. 
2 Memoir on Sheet 32 Geol. Surv. Scotland (1861), p. 42. The stratigraphical position of these 
“Houston Marls,” as they are locally called, is indicated in Fig. 155. 
3 Trans. Roy. Soc. Edin. vol. xxix. (1819) p. 515. 
