248 Mr. Williamson on the Limestones foiind 



filled with a variety of vegetable impressions, Ferns, Equisetoe, 

 &c., which will hereafter be described more fully. 



Some of the clays are compact, of a light blue colour, and 

 soon become decomposed on exposure to the weather. In- 

 stances of these will be found above and below the black bass. 



The most important seam to the geologist, excepting the 

 roofs of the Four-Feet Mine, is the black bass, so called from 

 its dark colour. The seam varies exceedingly, sometimes 

 being less than a foot, and at others two feet in thickness, but 

 appears to be regular in its position. It is of no use for burn- 

 ing, although it contains a small portion of lime. Its structure 

 is lamellar, resembling a dark shale, but rendered exceedingly 

 hard for the greater part of its thickness by the very large 

 proportion of sulphuret of iron it contains, the formation of 

 which seems to have been caused by the quantity of animal 

 remains found in connexion with it. This pyrites is generally 

 most abundant towards the upper part of the seam, where a 

 thin vein of it especially affords an indication of the presence 

 of an assemblage of remains offish. 



The number of freshwater shells and Entomostraca it con- 

 tains afford good evidence of its freshwater origin, which will 

 sufficiently separate the series from the magnesian limestone, 

 the remains from which are all marine. 



Immediately below the black bass is the seam of coal, ge- 

 nerally about six inches thick ; it is very impure, and wants 

 the compactness of the regular coal-seams : when burned, it 

 emits a strong sulphureous smell. 



Sect. VI. Series in the Bed of the Medlock. 

 In the bed of the river Medlock we cannot examine either 

 the main limestone or the black bass, as the former is con- 

 cealed under a considerable depth of water collected by a weir, 

 which in all probability rests upon the bass. Immediately 

 below the weir, the other limestones ai'e well exposed, with 

 their separating clays and sandstones, which slightly vary from 

 those of the Ardwick pits : I have not been able to detect the 

 seam of plants found in the Tunnel at the latter point, but here 

 we are able to trace the shales for a little distance above the 

 first limestone. Nothing however of interest presents itself, 

 except a thin seam of red marly clay, a few feet above the 

 limestone, which contains a number of Unionidcc. Between 

 the weir and the point where the water- works formerly stood, 

 is an elevation now quarried into for the sandstones, which 

 presents us with the section I have given of the series below 

 the limestones worked at Ardwick. It presents but two lime- 

 stones of a darker colour than those at the pits, but as one is 



