Here prevailing granite and diabase give rise to a complicated mountain system through which the rivers cleave their way in a curved and irregular course.
Slates, schists, quartzites and limestones form the greater part of the hills, but the Brocken and Victorshohe are masses of intrusive granite, and diabases and diabase tuffs are interstratified with the sedimentary deposits.
There occur also quartz-porphyry (Sierra Morena, Pyrenees, &c), diorite, porphyrite, diabase (well developed in the north of Andalusia, where it plays a great part in the structure of the Sierra Morena), ophite (Pyrenees, Cadiz), serpentine (forming an enormous mass in the Serrania de Ronda), trachyte, liparite, andesite, basalt.
Along Hudson Bay shore there is a strip of similar rocks, and a long row of small islands of the same age, with great sheets of trap or diabase forming the tops of the hills.
All the volcanic rocks of these islands are submarine stratified tuffs which are penetrated here and there by andesite or diabase dikes.