A few hornblendeschists are metamorphosed gabbros; others have developed from dikes or sills of lamprophyre.
The most fertile districts lie on the banks of the Elbe and near the North Sea, where, as in Holland, rich meadows are preserved from encroachment of the sea by broad dikes and deep ditches, kept in repair at great expense.
When it is remembered that the woodwork is infested by the pile worm (Teredo navalis), the ravages of which were discovered in 1731, the labour and expense incurred in the construction and maintenance of the sea dikes now existing may be imagined.
Neuwerk, containing some marshland protected by dikes, has two lighthouses and a lifeboat station.
Granite is the most widely spread of the crystalline rocks; but dikes of various kinds occur, and gneiss, schist and marble are also met with.