_Vulcanicity._--In intimate relationship with the mountain-building
orogenic crustal movements was the prevalence of volcanic activity
during the earlier part of this period. In the Lower Carboniferous
rocks of Scotland intercalated volcanic rocks are strikingly abundant,
and now form an important feature in the geology of the southern
portion of that country. Of these rocks Sir Archibald Geikie says:
"Two great phases or types of volcanic action during Carboniferous
time may be recognized--(1) Plateaus, where the volcanic materials
discharged copiously from many scattered openings now form broad
tablelands or ranges of hills, sometimes many hundreds of square miles
in extent and 1500 ft. or more in thickness; (2) Puys, where the
ejections were often confined to the discharge of a small amount of
fragmentary materials from a single independent vent." The plateau
type was most extensively developed during the formation of the
Calciferous Sandstone; the puy type was of somewhat later date. Basic
lavas, with andesites, trachytes, tuffs and agglomerates are the most
common Scottish rocks of this period. Similar eruptions, but on a much
smaller scale, took place in other parts of Great Britain.

Granites, porphyries and porphyrites belonging to this period occur in
the Saxon Erzgebirge, the Harz, Thuringerwald, Vosges, Brittany,
Cornwall and Christiania. Porphyrites and tuffs are known in the
French Carboniferous. In China, at the close of the period, there were
enormous eruptions of melaphyre, porphyrite and quartz-porphyry. In
North America, the principal region of volcanic activity lay in the
west; great thicknesses of igneous rocks occur in the Lower
Carboniferous rocks of British Columbia, and from the middle of the
period until near its close volcanoes were active from Alaska to
California. Igneous rocks of this period are found also in
Australasia.

_Climate_.--That the vegetation during this period was unusually
exuberant there can be no doubt, and that a general uniformity of
climatic conditions prevailed is shown not only by the wide
distribution of coal measures, but by the uniformity of plant types
over the whole earth. It is well, however, to guard against an
over-estimation of this exuberance; it must be borne in mind that the
physiographic conditions were peculiarly favourable to the
preservation of plant remains, conditions that do not