Of the Lower Palaeozoic rocks the Ordovician appears to be the most widely-spread.
From the Ordovician rocks of Esthonia, is in essential structure just the form demanded by comparative palaeontology to make a starting-point.
The Ordovician and Silurian systems are widely developed, and it is most probable that, with the exception of the Archean continents of Finland and the S, the sea covered the whole of Russia.
The Heteroptera can be traced back farther than any other winged insects if the fossil Protocimex silurica Moberg, from the Ordovician slates of Sweden is rightly regarded as the wing of a bug.
Upon the folded slates and schists which constitute these inliers the Devonian rests with marked unconformity; but north of the ridge of Condroz Ordovician and Silurian beds make their appearance.
Ordovician System.The succeeding Ordovician (Lower Silurian) system of rocks is closely connected with the Cambrian, geographically, stratigraphically and faunally.
On the Norwegian side the Cambrian is perhaps represented by the Roros schists which lie at the base of a great series of crystalline schists, the probable equivalent of Ordovician and Silurian rocks.
It has recently been stated, however, that the supposed Algae are in reality the megaspores of Vascular Cryptogams. Scarcely anything is known of Palaeozoic Florideae; Solenopora, ranging from the Ordovician to the Jurassic, resembles, in the structure of its thallus, with definite zones of growth, Corallinaceae such as Lithothamnion, and may probably be of the same nature.
In many places in the west they rest on what appear to be Ordovician beds, but without unconformity.
In the northern unfolded region great flows of basic lava lie directly upon the Cambrian and Ordovician beds of Siberia, but are certainly in part of Tertiary age.