164 GEOGNOSY. [Boox IL 
glass in them is no proof that they are not true tuffs; for the 
presence of these bodies depends upon the nature of the lavas. If 
the latter were not vitreous and microlithic, neither would be the 
tuffs derived from them. In the Carboniferous volcanic area of 
Central Scotland the tuffs are made up of débris and blocks of the 
basaltic lavas, and, like these, are not microlithic, though in some 
places they abound in fragments of palagonite (Fig. 27). 
Tuffs have consolidated sometimes under water, sometimes on 
dry land. As a rule they are distinctly stratified. Near the 
original vents of eruption they commonly present rapid alternations 
of finer and .coarser detritus, indicative of successive phases of 
volcanic activity. They necessarily shade off into the sedimentary 
formations with which they were contemporaneous. ‘Thus we have © 
tuffs passing gradually into shale, limestone, sandstone, &c. The 
intermediate varieties have been called ashy shale, tuffaceous shale, or 
shaley tuff, &e. From the circumstances of their formation, tuffs 
frequently preserve the remains of plants and animals, both terrestrial 
and aquatic. Those of Monte Somma contain fragments of land 
plants and shells. Some of those of Carboniferous age in Central 
Scotland have yielded crinoids, brachiopods, and other marine shells. 
Like the other fragmentary volcanic rocks, the tuffs may be sub- 
divided according to the nature of the lava from the disintegration 
of which they have been formed. Thus we have felsite-tuffs, trachyte- 




















































































Fic. 27.—Mioroscorio STRUCTURE or PaLaconiTe Turr rrom BuRNTISLAND, FIs. 
tuffs, basalt-tuffs, pumice-tuffs, porphyrite-tuffs, &c. A few varieties 
with special characteristics may be mentioned here.} 
Trass.—A pale yellow or grey rock, rough to the feel, composed of 
an earthy or compact pumiceous dust, in which fragments of pumice, 
trachyte, greywacke, basalt, carbonized wood, &c., are imbedded. 
It has filled up some of the valleys of the Hifel, where it is largely 
quarried as a hydraulic mortar. 
On the occurrence and structure of tuffs, see J. C. Ward, Q. J. Geol. Soc. Geilkie, 
Trans, Roy. Soc. Edin. xxix. Vogelsang, Z. Deutsch. Geol. Ges. xxiv. p. 548. Penck. 
op. cit. xxxi. p, 504. On the metamorphism of tuffs into lava-like rocks, see Dutton’s 
High Plateaux of Utah (U.S. Geogiaph. aud Geol. Survey of Rocky Mounts.), 1880, p. 79. 

