190 



GEOLOGY OF THE ISLE OF WIGHT. 



Fig. 72. 

 Cyclas Bj-istovii, Forbes. 



Fig. 73. 

 Unio Gibbsii, Forbes. 



in 



Forbes thus describes the Black Band : — 

 " It consists of nearly two feet of firm 

 carbonaceous laminated clays, abounding 

 in fossils. These are Paludina lenta, 

 very numerous; HydrohiaChasteli major ^ 

 scarce; Melanopsis carmata ; Limncea ; 

 Planorbis obtusus,o? large size ; a peculiar 

 small Cyclas, (C. Bristovii, Fig. 72), 

 which I have not met with elsewhere ; 

 and fish vertebraj. Impressions of the 

 linear leaves of gramineous plants, occa- 

 sionally large seed vessels, and Gyrogo- 

 nites are found in it, and lumps of 

 lignite. At its base is found a seam of 

 Unio (U. Gibbsii, Fig. 73) containing 

 well-preserved specimens." 



" The Black band rests in perfect conformity on a bed, three feet 

 thickness, of dark green marls, becoming paler below, and 

 separated by an irregular seam of broken univalves (Paludma 

 lenta) from greenish blue pale marly clays, with lenticular seams 

 of crushed Paludina. In the dark green marls are scattered fine 

 specimens of Paludina lenta and Melanopsis, also numerous fossil 

 bones. There are, moreover, in this bed, curious vertical or 

 slanting tubular concretions, with hollow cavities, as if formed 

 round the roots of plants." 



This weathering of the surface of the underlying Bembridge 

 Marls is very noticeable. It is a character still more marked 

 inland, where repeatedly after boring through unweathered 

 Hamstead Beds we penetrated a carbonaceous soil (the Black 

 Band), and then again entered weathered clays full of roots, like 

 ilie surface soil many feet above. 



Though this thin bed, however, can be traced nearly throughout 

 the Island, there seems to be no evidence of any real break. 

 Fossil species die out upwards one by one, and are replaced by other 

 species. Even the species which Forbes considered to be most 

 characteristic of the Hamstead Beds — Hydrobia Ghasteli — we have 

 shown in the last chapter not to be confined to this Series, but 

 to appear several feet down in the Bembridge Marls. Similarly 

 Nematura jmpa comes in somewhat higher : and so on with 

 others. Probably if the beds were now for the first time to be 

 sub-divided, we should class the the Bembridge Marls and the 

 greater part of the Hamstead Beds together, and separate the 

 marine beds as the commencement of a new series formed under 

 different conditions. 



But though no paljeontological break occurs at the Black Band, 

 it was so necessary to sub-divide the thick mass of clay above 

 the Bembridge Limestone, that some marked and easily re- 

 cognisable bed had to be traced. The Black Band proved to 

 be the only horizon that could be followed, and that would give a 

 satisfactory line from which to calculate dips and thicknesses. 



