Over the deepest valleys of the land in the interior this ice-cap must be at least 6000 or 7000 ft.
Across the ice-cap from Thule, work was started in the neighbourhood of St.
The islands are volcanic, the main geological formation being Tertiary or Jurassic basalt, which occasionally protrudes through the ice-cap in high isolated blocks near the shore.
The drainage of the interior of Greenland is thus partly given off in the solid form of icebergs, partly by the melting of the snow and ice on the surface of the ice-cap, especially near its western margin, and to some slight extent also by the melting produced on its under side by the interior heat of the earth.
The ice-cap of Greenland must to some extent be considered as a viscous mass, which, by the vertical pressure in its interior, is pressed outwards and slowly flows towards the coasts, just as a mass of pitch placed on a table and left to itself will in the course of time flow outwards towards all sides.