240 Transactions of the Royal Microscopical Society. 
With the exception of four or five thin bands of bedded ash, 
the whole thickness here given is made up of a succession of lava- 
flows. The thickness of the highest bed of ash is uncertain, since the 
ground is obscured by superficial deposits and the unconformably 
overlapping carboniferous limestone soon hides any further suc- 
cession eastwards. 
It is not always easy to determine the thickness of the separate 
lava-flows, though in many cases they differ somewhat in character 
from each other, and are frequently highly vesicular in their upper 
and lower portions. In some cases the vesicles have been most 
markedly drawn out in the direction of flow. On the whole, the 
varying character of the beds has given rise, by weathering, to a 
general step-like form of the hill side, reminding one of the origin 
of the word trap (¢appa, Swedish, a stair). 
The ash varies in character from a very fine close fragmentary 
rock to one made up of large angular fragments, and therefore 
more properly called a breccia. The colour is often a fine purple, 
though sometimes the finer ash has the same grey-blue colour as 
the lava. The latter is also sometimes of a rich purple, but is 
more generally some shade of green or dark blue. The various 
lava-flows are of all degrees of texture, from a very compact flinty 
rock to a finely crystalline and porphyritic one, in which the 
crystals of plagioclase are sometimes an inch in length. The band 
of Skiddaw slate interstratified among these lavas, and lying imme- 
diately beneath a finely porphyritic flow, is not more than six or 
eight feet thick, and is considerably hardened. 
2. Microscopie Examination. 
Perhaps the most satisfactory method of treating this part of 
my subject will be to take the various beds in succession, as given 
in the section, and describe the microscopic structure of each, where 
necessary. 
(1) Purple Ash.—This rock calls for no especial microscopic 
notice, being clearly fragmentary, and sometimes coarsely so. Such 
ashes when viewed under the microscope, in a thin slice, frequently 
show broken crystals and fragments of lava or previously formed 
ash-rocks imbedded in a fine dusty base, which last, under polarized 
light, with crossed prisms, usually appears dark with scattered 
points of light. 
(2) Lava.—Lithologically this rock presents a compact green 
base with yellowish or greenish tinge and small dark spots. It 
probably represents more than one flow, as it is about 300 feet 
thick, and has vesicular portions. 
Microscopically, the base consists of a mesh-work of minute 
felspar needles mingled with chlorite and magnetite, and a good 
