The third marine period had a long shallow-water phase at the commencement, with many minor oscillations, causing great variation in the character of the deposits and frequent minor unconformities. This shallow-water phase existed throughout Jurassic and Lower Cretaceous times. The deep-water phase existed during the deposition of the Upper Cretaceous deposits, and was succeeded by the second shallow-water phase, when the early Tertiary strata were accumulated.
The difference between the elevations which accompanied the Continental Periods and those which have been alluded to as minor elevations is no doubt one of degree, but in considering the British strata only no confusion is likely to arise on this account, as the difference was here very great.
The events which occurred during the continental periods are of extreme importance to the geologist. Every great upheaval was accompanied by crumpling and stiffening of portions of the earth's crust, and a definite trend was given to the strata as the result of these movements. It is to the earth-movements of the four great continental periods that the present structure of the British Isles is largely due, and in any attempt to restore the physical history of our islands considerable attention must be paid to the changes which were produced in the stratified rocks during these periods of earth-movement.
THE CAMBRIAN SYSTEM.
Classification. The rocks of the Cambrian system when found reposing on Precambrian rocks in Britain are always separated from the latter by an unconformity. The typical development of the rocks of the system, as the name implies, is in the hilly region of Caernarvonshire and Merionethshire in North Wales, and they are also well represented in South Wales, the border counties between England and Wales, and the North-West Highlands of Scotland. Two distinct classifications of the Cambrian rocks of Britain are in use, the original one founded on variations of lithological character, whilst the second depends upon faunistic differences, but the original lithological classification has been to some extent modified to make it locally correspond with the classification based upon palæontological grounds. The following table will shew the differences:—
| Lithological Classification. | Palæontological Classification. | |||
| Tremadoc Slate Series[64] | Beds with Intermediate Fauna | |||
| Lingula Flags Series | Beds with Olenus Fauna | |||
| Menevian beds (formerly included in Lingula Flags) |  | Beds with Paradoxides Fauna Beds with Olenellus Fauna | ||
| Solva beds Caerfai beds | | Formerly grouped together as Harlech or Llanberis beds | ||
[64] In accordance with the custom usually observed in Britain, the Tremadoc slates are placed in the Cambrian system; most continental geologists place them in the succeeding Ordovician system. The matter is not an important one, as the fauna is an intermediate one between that of the Lingula Flags and that of the Arenig series of the Ordovician system, and the beds are true beds of passage. As the lithological classification is essentially British, it will be as well to retain the Tremadoc Slates in the Cambrian system.