CHAPTER VII.

DYASSIC OR PERMIAN PERIOD.

1. Stratigraphical Evidence.

XCEPT in the north-east of England the Dyassic

Ex

rocks occur only in fragmentary strips and patches along the borders of the Coal-measure basins. This disconnected mode of occurrence is partly due to the overstepping of the Trias, under which many areas of Dyas are buried and concealed, and partly of course to the removal of large portions by erosion. Their limitation, however, to certain districts and their entire absence in the south of England are facts due to the conditions under which they were originally deposited. The Dyassic beds exhibit two distinct lithological facies, which may be called the eastern and western types; the latter being the more local and abnormal, while the former is similar to that which prevails in Germany.

A. The rocks of the eastern type are supposed to underlie the greater part of East Yorkshire, Lincoln, and Nottingham, and their outcrop forms a continuous strip of ground, between the Carboniferous and Triassic strata, from the coast of Durham, southward by Auckland, Ripon, and Pontefract, to the neighbourhood of Nottingham.

The beds are thickest and most purely calcareous in Durham, where they consist almost entirely of dolomite or magnesian limestone. The basement beds are soft sand and calcareous shale of variable thickness; the limestones

above are 450 feet thick, and the highest beds (proved in the borings at Middlesbrough) include deposits of rocksalt and gypsum.

In South Yorkshire the main mass of limestone is 270 feet thick, and there are marls and limestones above to a thickness of 140 feet; but in Notts all the beds are greatly attenuated, the total thickness near Mansfield being probably about 200 feet, of which not more than 100 are limestone, and parts of this are really calciferous sandstones. Near Nottingham it is still less, and seems to be on the point of thinning out when it is overlapped by the Trias.

A boring near Newark and about fifteen miles east of the outcrop at Mansfield is important as throwing much light on the lithological changes of the formation. This section proves that while the limestones remain about the same, the formation thickens eastward by an increase of the marls and shales, the calcareous marls at the base having a thickness of 118 feet, as compared with fifteen or thirty feet in Notts. A boring through the Trias at Owthorpe, in the south of Notts, proved the Dyas to be wanting there, and passed directly into Coal-measures. These facts show that, in Notts at any rate, the Dyas thins rapidly both southward and westward, whence we may conclude that land lay in both these directions, an inference which is confirmed by the collection of pebbles found in the basal breccia, which comprises not only pebbles of Carboniferous limestone and fragments of sandstone and shale from the underlying Coal-measures, but pebbles of slate, quartz, and quartzite, which have probably been derived from the rocks of Charnwood Forest.

B. Passing now to the western districts, we may select that of the Cumberland Plain or Vale of Eden as the most complete and important. Here and elsewhere on the west side of the Pennine chain there is very little lime

stone, and the rocks consist chiefly of red sandstones with occasional beds of calcareous breccia. Between Carlisle and Penrith there are two massive sandstones, the lower about 1,000 feet, and the upper about 1,500 feet thick, separated by a zone of red shales. Near Appleby the lower sandstone contains thick beds of breccia (locally called brockram), consisting of pebbles derived from the Carboniferous limestone, and the red shales have at their base bands of magnesian limestone and impure coal.

On the west side of the Lake District there is another strip of Dyas, but the lower (Penrith) sandstone is not present, and the basal beds at St. Bee's Head consist of magnesian limestone with fossils (11 feet thick) resting on a thin breccia of limestone fragments.

Northward, in Dumfries and Ayr, rocks similar to the Cumberland sandstones are found occupying several basinlike depressions, and resting partly on Carboniferous and partly on the older rocks. In Nithsdale and the valley of the Ayr sheets of porphyrite and volcanic ash are interstratified with the sandstones, and even the stumps of the volcanic vents from which these materials were ejected can be identified.

Returning to England, small detached patches of Dyassic deposits occur at intervals round the coalfields of Lancashire, Denbighshire, Shropshire, Staffordshire, Warwickshire, and Worcestershire. The Lancashire beds resemble those of Cumberland, consisting of a red sandstone overlain by red marls, shales, and limestones, with fossils of the Magnesian Limestone type.

Denbigh and North Staffordshire exhibit rather a different type, the beds being similar in both districts, and consisting of dark red and purple sandstones, with red marls and bands of cornstone; these beds are only slightly unconformable to the Upper Coal-measures.

In the Shrewsbury, Bridgenorth, and South Stafford

shire districts there is a similar set of beds, divisible into three stages, the middle one (from 200 to 400 feet thick) consisting of breccia and conglomerate, sometimes calcareous, and sometimes chiefly composed of felspathic materials-large blocks of felstone, syenite, and volcanic tuff occurring with boulders of quartz rock, limestone, sandstone, and slate; all these have probably been derived from the Archæan, Cambrian, and Silurian rocks that underlie the Dyas and Trias of the Midland counties.

Further south still the Lower Sandstones disappear, and at Church Hill, in Worcestershire, the breccia lies directly on the Coal-measures, the fragments composing it being very large and angular, and identifiable with rocks of Cambrian and Silurian age lying to the north-west. Small outliers of similar breccia rest against the Silurian of the Abberley Hills, and the most southerly is that at Haffield, at the southern end of the Malvern range.

This incoming of breccias southward with a thinning out of the lower beds is unequivocal evidence of a close approach to the southerly limit of the area in which these deposits were formed, and the breccias are also a proof that the shores were steep and rocky.

The only other districts in England where rocks of this age occur are, (1) Warwickshire, where they consist of red sandstones and marls, with occasional beds of calcareous breccia and conglomerate, the whole supposed to be nearly 2,000 feet thick, and (2) Anglesey, where similar beds to a thickness of 400 feet overlie the Coal-measures.

In Ireland three small patches occur in Ulster, fossiliferous magnesian limestone existing at Cultra, on Belfast Lough, and at Tullyconnel in Tyrone, and Boulder beds, with a limestone breccia at the base, rest on the Carboniferous Limestone of Armagh. The blocks in the Boulder beds are chiefly grits and sandstones derived from the Silurian and Old Red Sandstone districts that lie to the north-west.

2. Geographical Restoration.

In Chapter VI. the Carboniferous period was described as one of quiescence, during which the forces of terrestrial disturbance were in abeyance; but, as a calm precedes a storm in the atmosphere around our earth, and a great stillness often forebodes an earthquake, so in the earth's history a period of quiet deposition and rock-making has often been followed by a period of disruption and rockdestruction. Certain it is that the calm of Carboniferous times was followed by an epoch of great disturbance in the European and Atlantic areas, causing movements which produced very great geographical changes in the northern hemisphere, and resulted in the breaking up of the Carboniferous continents, and in the upheaval of the ground which had been covered by the Carboniferous seas.

At the close of the Carboniferous period there seem to have been important upheavals of land on either side of the great Atlantic continent. The Alleghany Mountains in America date from this epoch, as do also the series of domes, ridges, and faulted upheavals which make up the Pennine chain or "backbone" of England. It appears certain that the principal earth-throes, those which produced the more important disturbances of the Carboniferous rocks in Britain, occurred during the unrepresented period of time which intervened between the Coal-measures and the Dyas. The stratigraphical relations of the latter to the former make it clear that the disturbances, which bent the Coal-measures into the basin-shaped forms they now present, took place before any Dyassic strata were deposited. These movements resulted in the development of a double system of anticlinal and synclinal axes, one set running north and south, the other nearly east and west. It is impossible to say whether this double system of axes was formed simultaneously, or whether one set was