STRUCTURE OF ERUPTIVE ROCKS 209 
The mineral constituents also frequently show glass- and 
stone-inclusions, while liquid-cavities are relatively seldom 
seen. Now and again the lower part of a lava is crowded 
with indurated arenaceous and argillaceous matter, and 
contains occasionally well water-worn stones, as if the molten 
matter had flowed over the bed of the sea or of a lake or 
river, and thus caught up and enclosed some of the sedimentary 
materials lying in its path. Even fragments of trees have 
been found included in the basal portion of a lava—as in the 
case of a Carboniferous basalt-flow near Kinghorn, Fife. In 
all these respects effusive crystalline rocks differ markedly 
from intrusive rocks. As further differentiating lava-form 
rocks from sills, with which they might sometimes be con- 
founded, it may be noted that while the former may produce 
some induration of the rocks on which they rest, they never 
affect the overlying strata. Obviously, the superjacent beds 
have been deposited over the surface of the lava-form rock 
after consolidation had taken place, for the lines of bedding 
follow all the irregularities of the underlying rock-surface. 
When this is much rent and cleft, the cavities have been 
gradually filled up with sediment, while now and again 

FIG. 78.—EFFUSIVE IGNEOUS ROCKs. 
L, 1, lava-flows ; ¢, tuffaceous sandstones ; ¢!, tuffaceous shales. 
fragments of the scoriaceous crust of the old lava have been 
detached and enclosed in the immediately superjacent aqueous 
rock. Again, it may be noted that lava-form rocks are 
usually associated with stratified tuffs (see Fig. 78), 
O 


