The plateau formation of the country is not conducive to beauty, and its lovely sites must be sought in the valleys, and its wildest scenes on the coast. The deep cleft ravine of the Rance, the sweet valley of the Elorn, that of the Aulne, canalised, the Blavet, the Laïta and the Arz, will richly repay tracing upward.
The promontories of Crozon and Sizun till of late years were bare and untilled, and heath-grown; but the use of sardine heads as manure has given a great impetus to agriculture, and the demand for fir balks for the South Welsh mines has caused the planting of vast tracts with the Austrian pine.
The geological structure of Brittany is simple. It consists of an immense upheaval of granite through beds of Silurian and Cambrian schist. Rare deposits of lime occur in the folds of these beds. Dykes of quartz and diorite have traversed the schist and granite, and the face of the country is spotted with eruptions of igneous matter. It is as though the crust had been full of blowholes through which the molten diorite had rushed to the surface. The presence of quartz or diorite in the neighbourhood can always be recognised by the employment of one or the other to metal the roads.
The granite extends from the bay of Mont Saint Michel to the extreme point of Finistère and reappears in the isles beyond; it is interrupted only here and there by the sedimentary beds. The Châteaulin district, however, and the basin between the prongs of the mountain fork, are all of Cambrian and Silurian beds. But from the Pointe du Raz the granite extends almost uninterruptedly to the Rhone.
The Brittany granite is for the most part fine grained and soft, so that it lends itself easily to be carved, and has been freely employed in churches and secular buildings from the 11th century. But it is readily corroded by the weather, and this has given to denuded surfaces a smooth and rounded shape, and has taken the angles off exposed masses that form tors, and has occasioned the fall of many into utter ruin.
A band of syenite runs from near Lamballe to Cap Fréhel, where it forms magnificent cliffs. Syenite again comes to the surface at Trégastel and on the coast north of Morlaix. The Monts d'Arrée are of Cambrian schist and furnish slates here and there of good quality. Taking a section across the inner basin, the granite is quitted at Plounéour, then the ridge of Cambrian schist is reached, after crossing the culminating point of S. Michel, which is of Cambrian sandstone; when we reach S. Herbot we are on Silurian beds. Continuing our course south, the sandstone makes way for slaty schists, and to this succeeds the grauwacke of Brasparts. The Montagnes Noires belong to the Silurian system.
The Kersanton stone, so extensively employed for figure and foliage sculpture in Lower Brittany, is an amphibolite with mica freely comminuted and distributed through the substance. It is very dark in colour, and hardens with exposure. It comes from quarries to the south of the Rade de Brest.
An interesting deposit is the tertiary limestone of S. Juvat beside the Rance. It is of no great extent, but is of vast commercial importance. The bed is composed of an agglomerate of shells and bones. In places it lies under a deposit of as much as 45 ft. of sand. It is a veritable mine of wealth in a country so destitute of lime as is Brittany.
A mineralogical curiosity is the staurotides found at Baud, Scaer, and in various places about the Blavet. The peasants attach a superstitious value to them as marked with the cross, and in some they affect to recognise the nails. They are often sold on stalls at a Pardon. They are formed by trapdykes that have penetrated the schist, and fused and run together some of its constituents, which have afterwards crystallised, sometimes as parallel prisms, at others as set transversely forming the ordinary or the S. Andrew's Cross.