- a - - = 
_ 
566 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boor IV. 
solidated at some depth, and therefore whether or not it is to be 
regarded as evidence of an actual volcanic outbreak at the locality, 
he is not liable to the same uncertainty among the fragmental 
eruptive rocks. Putting aside the occasional brecciated structure seen 
along the edges of plutonic intrusive masses, he may regard all the 
truly fragmental igneous rocks as proofs of volcanic action having 
tre3 og ah 
‘ ’ 
» 
been manifested at the surface. The agglomerate found in a 
volcanic neck could not have been formed unless the vapours in the 
vent had been able to find their way to the surface, and in so doing 
to blow into fragments the rocks on the site of the vent as well as 
the upper part of the ascending lava-column.* Wherever therefore 
a bed or a series of beds of tuff occurs interstratified in a geological 
formation it points to contemporaneous volcanic eruptions. Hence 
the value of these rocks in interpreting the volcanic annals of a 
region. | 
The fragmentary ejections from a volcano or a cooling lava- 
stream vary from the coarsest agglomerate to the finest tuff, the 
coarser materials being commonly found nearest to the source of 

Fic. 294.—EszoTEep Voutcanic Buock (12 x 15 x 17 inches) In Lower CaRsonrrEeRous 
SHALES, PETTycuR, Fire. 
discharge. They differ in composition, according to the nature of 
the lavas with which they are associated and from which they have 
been derived. ‘Thusa region of trachyte-lavas, supplies trachyte-tuffs 
and trachyte-breccias; one of basalts gives basalt-breccias, basalt- 
agglomerates, basalt-tuffs; one of obsidians yields pumiceous tuffs 
and breccias. ‘The fragmentary matter ejected from volcanic vents, 
has fallen partly back into the funnels of discharge, partly over the 
surrounding area. It is therefore apt to be more or less mingled 
with ordinary sedimentary detritus. We find it indeed passin 
insensibly into sandstone, shale, limestone, and other strata. Alterna- 
tions of gravelly peperino-like tuff with a very fine-grained “ash ” 
may frequently be observed. Large blocks of lava-form rock, as 
well as of the strata through which the volcanic explosions have 
taken place, occur in the tuffs of most old volcanic districts. Occa- 
sionally such ejected blocks or bombs are found among fine shales 
1 It is conceivable that where a mass of lava was injected into a subterranean 
cavern fragmentary discharges might take place and partly fill that cavity ; but such 
exceptional cases are probably rare. 
