
54 STRUCTURAL AND FIELD GEOLOGY 
Volcanic Breccia is a mass composed of angular fragments 
of volcanic rock, Volcanic Tuff is the name given to 
aggregates of the finer grained ejectamenta (Plates XV.; 
XX. 1). These are often arranged in layers and beds which 
have been spread out by water action. There are endless 
varieties of structure and texture—some tuffs consisting of 
lapilli, or of grit, or of sand and ash; others made up of all 
four, and not infrequently arranged in lenticular layers and 
beds. Some volcanic tuffs consist of the finest ash-like 
material, forming a dull, fine-grained or compact rock, which 
varies in colour from white or grey, to darker or lighter 
shades of red, blue, yellow, etc. 
As tuffs are composed almost exclusively of fragments and 
the comminuted débris of lava, they naturally differ in char- 
acter according to that of the rocks from which they have 
been derived. Hence we have Jdasalt-tuff, andeszte-tuff, 
rhyolite-tuff, etc., any of which may of course contain a large 
or small proportion of fragments and débris of sedimentary 
rocks. Frequently, however, the latter are entirely absent. 
