Or 
— 
Iw 
PROFESSOR GEIKIE ON THE 
Lr THE FRAGMENTAL Riocks of DLurnrs, 
(Plate XII. fig. 10.) 
From the nature of their origin the fragmental volcanic products cannot, 
like the crystalline rocks, be grouped into very definite petrographical sub- 
divisions. They are not chemical mixtures, but mere mechanical aggregates, 
liable to constant variation in the characters and proportions of their con- 
stituents. Thus on the large scale we may encounter one of these masses 
presenting the greatest contrast in the composition even of two adjacent 
portions ; and even when examined with the microscope, similar extreme 
diversity and variety may be traced. 
1. General External Characters and Modes of Occurrence.—In the first 
place, it is to be observed that the fragmental rocks have two distinct modes of 
occurrence, in each of which they present special petrographical characters. 
They occur (a) fillimg up volcanic vents, and (6) interstratified with bedded 
lavas or with strata of an ordinary sedimentary kind. 
(2) In Volcanic Vents.—By far the coarsest and most tumultuously assorted 
varieties occur in this position, Large subangular or somewhat rounded blocks 
of sandstone, limestone, or other stratified rock, according to the nature of the 
surrounding strata, are commingled with abundant blocks of dolerite, basalt, or 
some other variety of igneous rock in an earthy and gravelly paste of the same 
materials still further comminuted. These agglomerates are for the most part 
quite unstratified, though sometimes traces of a rude bedding may be dis- 
cerned among them, the layers standing on end or at high angles in the 
manner already described (ante, p. 463). The agglomerate of Arthur Seat is a 
well known and excellent example. In some cases the stones are remarkably 
angular, giving the rock the character of a breccia, though this variety is much 
less frequent than the preceding. The fragmental detritus in the vents is often — 
a dull, dirty-green gravel, partially cemented in an incoherent paste of the same 
composition. The small stones of the gravel consist chiefly of varieties of dolerite 
or basalt, usually much decayed. Larger blocks of the same rocks, as well as 
of sandstones, limestones, shales, &c., are scattered abundantly through the 
mass. ‘This is the general character of the tuff filling up the vents in Fife. In 
districts where the lavas erupted have been porphyrites, the voleanic agglome- 
rates and tuffs consist of the debris of these rocks. Fragments of older tuff 
may constantly be detected among the materials in the vents. The probable 
meaning of this fact has been already stated (ante, p. 461). 
(>) In Interstratified Sheets.—The showers of dust, sand, and lapilli ojoctalll 
from volcanic vents falling upon lakes, rivers, or the sea, sink to the bottom of 
the water, where they accumulate in layers, more or less mixed with the 
ordinary sand, mud, or other deposit. In proportion, therefore, to the vigour 
