Page:VCH Lancaster 1.djvu/67

GEOLOGY The Middle Measures of the Bolton, Oldham, and Rochdale districts have yielded a number of rare and interesting crustaceans; and work which is now being carried on at Sparth Bottoms near Rochdale in shales over the Arley Mine bids fair to reveal many new forms.

Fossil fishes are represented by large spines, teeth, scales, and not rarely by whole fishes. Seven species of Pleuracanthian spines are known, the commonest being Pleuracanthus lævissimus. All these spines consist of a stout bony rod which was imbedded in the body muscles of the fish, either behind the head, or in front of each of the paired fins. The hinder surface of each spine is armed with small acutely pointed denticles arranged in two or even more rows. Larger and stouter spines, often a foot in length, and ornamented by oblique rows, or tubercles, have received the name of Gyracanthus, whilst spines ornamented by longitudinal ridges are known as Sphenacanthus. Many of the Lower Coal Measure fishes continue to exist, and the black shales forming the roof of several of the thick coal seams are veritable storehouses of fish remains. Amphibia are not satisfactorily represented, although certain large ring vertebræ and small ribs are known and doubtfully referred to Archegosaurus.

UPPER COAL MEASURE FAUNA

As already stated, the fossils found here are scanty and small. The period was one in which extinction was going on.

Spirorbis pusillus, and ostracods, the latter of several species, alone seemed to have flourished in anything like numbers, whilst Anthracomya Phillipsii, and ''A. lævis var. Scotica'' are the only common forms amongst the mollusca. Arthropods are represented by Estheria tenella, and ''Leaia Leidyi var. Williamsoniana''. The fishes were mainly Pleuracanthus, Gyracanthus, Ctenodus Murchisoni, and Megalichthys Hihherti, a typical Middle Measure form. Small phalanges referred to Labyrinthodon were found by Mr. Chas. Roeder at Longsight and are the only remains of amphibia known from these beds.

PERMIAN

Strata belonging to this period formerly occupied a much greater area in the county than now, the formation having been extensively swept away by post-Permian denudation, which was possibly rendered more effective by a considerable amount of earth movement, such as faulting. (See p. 20.) Much of the existing Permian strata is covered by the Trias or Glacial Drift, so that the only portions readily accessible are small and comparatively unimportant. A narrow band of Red Sandstones, Marls, and Limestones borders the South Lancashire Coalfield from Sutton near St. Helens, Edge Green, Leigh, and Astley to Eccles. At the latter place, and again at Salford and Cheetham Hill, the formation has been faulted northwards by the Great Irwell Valley and other faults, which here traverse the coalfield. The fact that the Permian has shared in the movement and faulting of the coal measures indicates that the latter movements took place after the deposition of Permian and not before.

The Red Sandstones, Marls, and Limestones are best seen on the east side of Manchester from Collyhurst to Stockport. They were also cut through in the making of the Fallowfield and Burnage section of railway on the east of Manchester, where their junction with the Coal Measures below could also be seen. Small patches of the Permian sandstones occur west of Preston, on the banks of the Ribble near Clitheroe, on the Ingleton Coalfield, and in the Furness district. At this latter place the beds are much obscured by drift.

The position of the Permian has been determined in a number of cases by means of borings made in search of iron ore. The Red Sandstones are seen at High Cocken, north of Barrow, and quarried at Hawcoat, whilst old quarries opened in the same rock exist in the grounds of Furness Abbey. The Magnesian Limestone which underlies the sandstone is also present in the Furness district, and has been worked at Old Holebeck. The smaller patches which occur near Clitheroe and elsewhere owe their preservation to their position on the downthrow side of faults. They are outliers of the great mass of Permian strata which formerly existed.

Fossils are poorly preserved in the Permian sandstones and marls, the latter yielding at various places examples of Schizodus and Bakevellia, whilst the thin limestones are at times crowded with species of Rissoa, Turbo, etc. Polyzoa are not unfrequent in the Magnesian Limestone.

The Permian System as a whole consists of the following divisions:—

Only the Upper Red Sandstone and Magnesian Limestone are exposed in the county. 23