THE SEVERN AND ITS TRIBUTARIES. 615 



probably been formed from mechanical sediments derived from the 

 denudation of Cambrian limestones, such as those of Bala and 

 Hirnant. 



Hence in discussing the source of these constituents of the Severn 

 ooze, three possibilities present themselves for investigation. The 

 marine organisms may have been derived (i) from the older for- 

 mations through which the Severn and its tributaries flow; or (ii) 

 from the alluvial flats of its estuary ; or finally (iii) from the coast of 

 the Bristol Channel, where under true marine conditions organisms 

 which could furnish such structures as we have described are known 

 to flourish abundantly. 



To commence with the first suggestion. Although it may seem 

 improbable that the older formations should have furnished any 

 considerable portion of the organic remains, yet to set the matter at 

 rest, I visited the Severn and two of its tributaries, the Bristol and 

 Stratford Avons (which seemed most likely sources, since they 

 flow through Secondary formations), and obtained from them 

 samples of mud at points above the limit of tidal influence. The 

 locality selected on the Severn was about two miles above 

 Worcester ; on the Stratford Avon at DefTord ; and on the Bristol 

 Avon, at Saltford and Keynsharn. In every sample sponge-spicules 

 occuried pretty freely, but a close examination proved to a certainty 

 that these were all fluviatile, and not marine ; they not only agreed 

 in size and shape with spicules of Spongilla fluviatilis, but were 

 sometimes associated with the characteristic spicules of the stato- 

 blasts, or even with the entire statoblast itself. This observation 

 was interesting, as showing not only the entire absence of marine 

 forms, but also a wider distribution of freshwater sponge-spicules than 

 I had previously supposed. Not only were marine spicules absent, 

 but there were also no traces of Foraminifei a, coccoliths, or any other 

 marine remains, such as are present in the tidal silt. Thus the 

 rivers, as a possible source of these remains, are eliminated. 



The chief constituents of the silt at Worcester are fragments of 

 quartz with some little flint ; at DefTord similar fragments, together 

 with rhombohedra of calcite, both separate and aggregated into 

 masses like sugar candy, as well as Diatoms and spicules of Spongilla 

 jluviatilis : at Saltford also fragments of quartz containing minute 

 cavities, minute crystals of quartz, some flint, occasional flakes of mica, 

 many spicules of Spongilla Jluviatilis, and Diatoms. 



We have next to inquire into the second possibility, i. e., the 

 chance of derivation from the ancient alluvium of the Severn ; this 

 is not so readily dismissed. The blue silt of the Severn alluvium is 

 strikingly similar in all its characters to the modern ooze; it consists 

 of a similar admixture of mud and angular siliceous fragments ; while 

 marine sponge-spicules, Foraminifera, coccoliths, and other marine re- 

 mains similar to those of the modern silt, are universally disseminated 

 throughout its mass. They occur in its upper portion where it forms 

 the shore of the estuary, and on the surface of fields where it is tilled ; 

 and they are just as plentiful deeper down, 15 or 20 feet below the 

 surface, as in the new cutting for the railway to the Severn tunnel, 



Q.J.G.S. No. 156. 2x 



