chap, in 
ORIGIN OF TUFFS 
33 
In the vast majority of cases, the fragmentary substances ejected by 
ancient volcanic explosions, like those of the present day, have consisted 
wholly or mainly of material which existed in a molten condition within 
the earth, and which has been violently expelled to the surface. Such 
ejected detritus varies from the finest impalpable dust or powder up to 
huge masses of solid rock. These varioris materials may come from more 
than one source. Where a volcanic orifice is blown out through already 
solidified lavas belonging to previous eruptions, the fragments of these lavas 
may accumulate within or around the vent, and he gradually consolidated 
into agglomerate or breccia. Again, explosions within the funnel may 
break up lava-crusts that have there formed over the cooling upper surface 
of the column of molten rock. Or the uprising lava in the chimney may 
i'e spurted out in lumps of slag and bombs, or may he violently Idown out 
ill the form of minute lapilli, or of extremely fine dust and ashes. 
Althougli in theory these several varieties of origin may lie discriminated, 
it is hardly possible always to distinguish them among the products of 
ancient volcanic action. In the great majority of cases old tuffs, having 
been originally deposited in water, have undergone a good deal of decom- 
position, and such early alteration has been aggravated by the subsequent 
influence of percolating meteoric water. 
Where disintegration has not proceeded too far, the finer particles of 
tuffs may be seen to consist of minute angular pieces of altered glass, or of 
inicrolites or crystals, or of some vitreous or semi-vitreous substance, in 
which such inicrolites and crystals are enclosed. It has already been stated 
that the occurrence of glass, or of any substance which has resulted from 
the devitrification of glass, may he taken as good evidence of former volcanic 
activity. 
i\Iost commonly, especially in the case of tuffs of high anticpiity, like 
those associated with the Pakeozoic formations, the fresh glassy and micro- 
htic constituents, so conspicuous in modern volcanic ashes, are hardly to he 
Recognised. The finer dust which no doubt contained these ehai’acteristic 
substances has generally passed into dull, earthy, granular, or structureless 
material, though here and there, among basic tuffs, remaining as palagonite. 
hi the midst of this decayed matrix, the lapilli of disrupted lavas may 
uudure, hut it may lie difficult or impossible to decide whether they were 
derived from the breaking up of older lavas by explosion, or from the 
blowing out of the lava-crusts within the funnel. 
The cellular structure, which we have seen to be a maiPedly volcanic 
peculiarity among the lavas, is not less so in their fragments among the 
'''■ogloinerates, breccias and tuffs ; indeed it may he said to be eminently 
eharacteristic of them. The vesicles in the lapilli, bombs, and blocks are 
sometimes of large size, as in masses of ejected slag, hut tliey range down 
to niicroscopic minuteness. The lapilli of many old tuffs are sometimes 
‘30 crowded with such minute pores, as to show that they were originally 
true pumice. 
Phe composition of tuffs must obviously depend upon that of the lavas 
VOL. 1 D 
