1866.| _ Hints to Home Tourists. 356 
or on the land, and there enveloped the remains of plants and animals. 
When no trace of any ash is to be found, search must be made 
among the trap-rocks for proof that they were lava-flows, and not 
masses of melted matter which consolidated far beneath the surface. 
In the latter case they would not be volcanic, and their geological 
age might not be fixed, nor would they present the same variety of 
interest which distinguishes the truly volcanic rocks. If the trap 
does not occur in an amorphous mass, but is arranged in beds, there 
is some reason to suspect that it may have been erupted at the 
surface. If, moreover, the beds are found to differ in structure and 
texture from each other, this suspicion is considerably strengthened, 
and if the upper and under portions of a bed present a vesicular 
slagey appearance, it. may be concluded with tolerable certainty that 
the bed in question is an old lava-flow. When the trap occurs 
abundantly, it is usually not by one character, but by a number of 
convergent proofs that we determine it to be of voleanic origin. 
Having satisfied ourselves that the rocks are relics of former 
voleanic phenomena, it remains to determine their geological age. 
When we look at a hard black basalt, a coarsely crystalline dolerite, or 
dull compact blue clinkstone, or a dark glass-like pitchstone, we can 
easily admit that they probably each belong to different eruptions. 
To one unacquainted, however, with the accuracy of geological 
research, it may seem well-nigh incredible that we should be able 
to arrange a true chronological series out of what seems involved 
from its very nature in hopeless confusion. He may find some 
difficulty in conceiving how it can be possible to pronounce with 
confidence that a certain chain of hills of volcanic rocks is older than 
some other chain; that one special hill in a district is younger than 
its neighbours ; nay, that even one part of a single hill was erupted 
long ages after the other parts. And yet all this and more can be 
done very easily and with confidence. Snowdon, for instance, is 
built up of memorials of volcanoes immensely older than those of 
the Derbyshire hills ; the rocks of the Sidlaw hills of Forfarshire 
are likewise far more ancient than those of the Lothians ; the ashes 
and traps of Wexford belong to a time vastly anterior to those 
of Limerick, and the upper portions of Arthur Seat, at Edinburgh, 
were ejected many a long age after the volcano that gave birth to 
the lower portions of the hill had become extinct. The deter- 
mination of these relative dates is really a matter of extreme sim- 
plicity. By the well-known geological laws of superposition and 
organic succession, the age of a group of stratified rocks is fixed, 
and if such a group contains an intercalated series of volcanic 
rocks, it is clear that these must belong to the same geological 
period. Thus in the Snowdon district the stratified rocks contain 
fossils which show them to be of Lower Silurian age, and hence 
the trappean rocks interbedded with them must be the products of 
