210 STRUCTURAL AND FIELD GEOLOGY 
Flows vary in thickness, some being only a few feet, 
while others attain a depth of many yards. The more basic 
lavas generally preserve a somewhat equable thickness, the 
intermediate and acid kinds tending rather to be irregular, 
so that they thicken and thin-out more or less rapidly. 
(0) Pyroclastic or Fragmental Effusive Rocks.—The 
tuffs usually associated with lava-form rocks vary in character. 
As might have been expected, their dominant ingredients 
consist of the comminuted débris and larger fragments of 
the lavas they accompany. Thus we have basalt-tuff, 
andesite-tuff, trachyte-tuff, etc. All varieties of texture and 
structure are met with, some rocks being very fine grained, 
while others are mere aggregates of lapilli and blocks—finer 
and coarser grained materials often rapidly alternating in 
a vertical section. Bedding is usually pronounced—many 
of the finer tuffs being beautifully laminated. Occasionally, 
very large sporadic blocks may be encountered in a bedded 
mass of small lapilli, and generally increase in numbers 
as the old focus of eruption is approached. Tuffs are fre- 
quently interstratifed with ordinary sedimentary beds, and 
when such is the case the tuffs themselves usually contain 
a larger or smaller proportion of arenaceous or argillaceous 
materials, and thus frequently graduate into sandstone and 
shale. Fossils may be included not only in the sedimentary 
beds associated with tuffs, but in the tuffs themselves. 
Fragments of plants, and various marine organic remains, 
for example, not infrequently occur in the tuffs and tuffaceous 
sandstones and shales, which are associated with the andesitic 
lavas of the Carboniferous system in Scotland. 
Mode of Occurrence of Effusive Rocks.—Sometimes a 
flow, with its accompanying tuff, occurs singly; more usually, 
however, flows and tuffs appear in consecutive series. Some 
effusive rocks, occupying a limited area, are obviously the 
products of an, isolated volcano. Others extend over very 
wide regions, and appear to represent the products of a series 
of more or less closely associated foci of eruption, the 
successive lavas and tuffs discharged from the several vents 
interosculating and overlapping. A good example is furnished 
by the eruptive rocks of the Sidlaw and Ochil Hills, some 
of the old vents from which these were discharged being 


