Two systems. At the close of the Cretaceous period great eruptions of basalt and basaltic tuff took place, especially in the Lisbon area.
The lavas are usually porphyrites, which occur in sheets, with intercalated bands of volcanic tuff that are sometimes strongly felsitic. One of the vents by which such materials were ejected occurs in the Braid Hills on the south side of Edinburgh.
Large areas are overlain with trachyte, basalt, obsidian, tuff and pumice.
Where lava has been piled up in successive nearly horizontal sheets, with occasional layers of tuff or other softer rock between them, it offers conditions peculiarly favourable for the formation of escarpments, as in the wide basalt plateaus of the Inner Hebrides.
In the Rudolf province there and are the basalt, lava, tuff and kenyte of the volcanic mineralogy.
At the base there is frequently a conglomerate or tuff of porphyritic rocks.
In the vicinity of the Ullughmuz-tagh there exist numerous indications of former volcanic activity, the eminences and summits frequently being capped with tuff, and smaller fragments of tuff are scattered over other parts of the Arka-tagh ranges.
Occasional beds of tuff are intercalated among these lavas, and likewise seams of fine clay or shale which have preserved the remains of numerous land-plants.
The remarkable stability of the mountain appears to be due to the innumerable dikes which penetrate the lava flows and tuff beds in all directions and thus bind the whole mass together.
The latter are represented by large contemporaneous deposits of tuff and felsitic lava which in the Snowdon District are several thousand feet thick.