Vol. 68.'] SUCCESSION IN THE NORTH-WEST OF ENGLAND. 475 



(/^) The Lower Dibiinophyllum STib-zone. 



This sub-zone is notable for its great thickness, for the purity of 

 its limestones, and for its uniformity throughout. It represents 

 the period of greatest submergence in the N^orth- Western Province,, 

 and consequently presents very similar characters throughout the 

 different districts. It can also be readily correlated with the beds 

 of the corresponding age in the South- Western Province; and, as 

 there can be no doubt regarding the contemporaneity of this 

 sub-zone with the Lower DihunophyJlum Sub-zone of the Bristol 

 District, we may adopt Dr. Yaughan's nomenclature without fear 

 of causing confusion in the future. 



Lithology. — The sub-zone is characterized over the greater part 

 of the JS^orth- Western Province by the occurrence of somewhat 

 massive grey and dove-coloured limestones, and by the complete 

 absence of shaly partings or sandy intercalations. The rock is made 

 up entirely of foraminifera and microscopic fragments of calcareous 

 organisms in which the larger fossils are embedded. The sub-zone 

 is remarkable for the development of certain structures, namely ,^ 

 (1) ' spotted ' beds and (2) ' pseudo-breccias.' These occasionally 

 pass into one another, but are usually distinct. 



The 'Spotted' Beds. 



These are especially characteristic of the base and summit of the 

 sub-zone. They consist of small spherical patches of darker lime- 

 stone surrounded by a lighter matrix, the dark spots averaging an 

 inch in diameter. At a distance, therefore, a broken surface re- 

 sembles a conglomerate rather than a breccia : on closer inspection, 

 however, the edges of the dark spots seem to fade gradually inta 

 the matrix, and are evidently not of fragmental origin, both portions 

 of the bed having been deposited contemporaneously. The spots- 

 are not typical concretions : for they exhibit neither concentric nor 

 radial structure, and do not separate readily from the matrix on 

 weathering. Again, in the majority of examples analysis shows- 

 no marked or constant difference between the chemical composition 

 of the spots and that of the matrix. This is especially the casa 

 with the spotted beds which occur so constantly near the base of the 

 sub-zone. Microscopic examination throws but little light on the 

 origin of the spots : sections show that portions of the rock are 

 richer in foraminifera than others, and sometimes these organisms 

 seem to be more densely packed in the spots than in the matrix ; 

 but this arrangement does not by any means obtain generall}", 

 and appears to be accidental. So far as can be ascertained, the 

 spots are due to a slight tendency to concretionary action, but 

 this action has been practically confined to the coloured impurities 

 in the original deposit, which have been concentrated locally to 

 form the dark spots, leaving the bulk of the rock nearly free from 

 colouring-matter. Since the rock is almost a pure limestone, the 

 amount of material capable of being affected by concretionary 

 action must have been very slight. The foregoing remarks apply 



