Chapter XI

Intercalation of coal seams among mountain limestone beds of
Mid-Lothian--North Greens seam--Most of our coal seams
indicate former land-surfaces--Origin of coal a debated
question--Erect fossil trees in coal-measures--Deductions to
be drawn therefrom--Difference between the mountain limestone
of Scotland and that of England--Coal-bearing character of the
northern series--Divisions of the Mid-Lothian coal-field--The
Edge coals--Their origin illustrated by the growth of modern
deltas--Delta of the Nile--Of the Mississippi--Of the
Ganges--Progress of formation of the Edge coals--Scenery
of the period like that of modern deltas--Calculations of
the time required for the growth of a coal-field--Why of
doubtful value--Roslyn Sandstone group--Affords proofs of
a general and more rapid subsidence beneath the sea--Its
great continuity--Probable origin--Flat coals--Similar
in origin to the Edge coals below--Their series not now
complete--Recapitulation of the general changes indicated by
the Mid-Lothian coal-field,

204

CHAPTER XII.

Trap-pebbles of the boulder--Thickness of the earth's crust
unknown--Not of much consequence to the practical
geologist--Interior of the earth in a highly heated
condition--Proofs of this--Granite and hypogene
rocks--Trap-rocks: their identity with lavas and
ashes--Scenery of a trappean country--Subdivisions of
the trap-rocks--Intrusive traps--Trap-dykes--Intrusive
sheets--Salisbury Crags--Traps of the neighbourhood of
Edinburgh--Amorphous masses--Contemporaneous trap-rocks of
two kinds--Contemporaneous melted rocks--Tests for their age
and origin--Examples from neighbourhood of Edinburgh--Tufas
or volcanic ashes--Their structure and origin--Example
of contemporaneous trap-rocks--Mode of interpreting
them--Volcanoes of Carboniferous times--Conclusion,

235

THE STORY OF A BOULDER.