GEOLOGY* 
73 
Volcanoes and Volcanic BocJcs .^-It is almost needless to say that any 
additional information on the distribution of volcanic vents, recent or 
extinct, is of interest. In the case of extinct vents, the geological date 
of the last eruptions should be ascertained if practicable. This may 
sometimes he determined by finding organic remains or sedimentary beds 
of known age interstratified with the ashes or lava-streams near the base 
of the volcano. 
Coasts .—The subject of the erosion of coasts is now fairly understood, 
and there is no doubt that the relative importance of this form of denuda¬ 
tion was greatly overrated by many geological writers, who took their ideas 
of geological denudation generally from the phenomena observed in the 
islands, and on some of the coasts of Western Europe. Still, wherever 
cliffs occur, they afford good sections, and deserve examination. One 
question will usually present itself to almost every geological observer, 
and that is, whether any coast he may be landing upon affords evidence 
of elevation or depression. In the former case, beds of rolled pebbles or 
of marine shells, simitar to those now living on the shore, may be found 
at some elevation above high-water mark. Very often the commonest 
molluscs in raised beds are the kinds occurring in estuaries, which are 
different from those inhabiting an open coast. Caution is necessary, how¬ 
ever, that heaps of shells made by man, or isolated specimens transported 
by animals (birds or hermit-crabs), or by the wind, be not mistaken for 
evidence of raised beds. If the shore is steep, terraces on the hill-sides 
may mark the levels at which the sea remained in past times, but some 
care is necessary not to mistake outcrops of hard beds for terraces. If 
dead shells of species of moll us ca, only living in salt-water estuaries, are 
found in places now beyond the influence of the tide, it is a reasonable 
inference that elevation has taken place. 
The evidence of depression, on the other hand, unless there are buildings 
or trees partly sunk in the water, is much less readily obtained, and 
neither trees nor buildings are available as evidence, unless the depres¬ 
sion is of comparatively recent date. The best proof is the form of the 
coast. If deep inlets of moderate breadth occur, with numerous branches, 
a little examination will frequently show whether such inlets are valleys of 
subaerial erosion, as they not unfrequently are, that have been depressed 
below the sea. A good and familiar example of such a depressed valley 
is to be found at Milford Haven in South Wales. In higher latitudes, 
