Vol. 59.] A LOTVEE-GEEENSAND FOSSILIFEEOUS BAND. 24B 



have been of appreciable length ; and meanwhile the fauna of any 

 particular portion of the sea-floor may to some extent have 

 responded to the changing conditions. 



Condition of the Fossils. 



Despite the unmistakable evidence for strong current-action during 

 the accumulation of the sands, we find with some surprise that 

 the shells embedded in the limestone, although fragile, are in 

 splendid preservation, and rarely show even the slightest trace of 

 abrasion.. The delicate ornamentation of the striated and pustulated 

 brachiopods, and of the pectens and other lamellibranchs, is gene- 

 rally as fresh and sharp as the most fastidious palaeontologist could 

 desire. Both valves are nearly always present and in position ; and 

 it is clear that the molluscs have flourished at the site where their 

 shells now occur. In this respect the bed differs greatfy from the 

 Faringdon ' Sponge-Gravels ' and from the Potton ' Phosphate-Bed,' 

 in which nearly all the materials show signs of transportation ; but 

 it somewhat resembles the fossiliferous beds at Brickhill and at 

 Upware, where many of the delicate shells are similarly, though 

 not quite so perfectly, preserved and at the same time associated 

 with much current-drifted detritus. To explain this anomaly, we 

 may suppose that the molluscs have lived in clusters on the sea- 

 floor during intervals when the strength of the currents at this 

 particular spot was abated; and that a gritty calcareous mud was 

 rapidly formed by the decay of the more perishable shells and other 

 organisms, and thus provided a matrix which quickly became 

 sufficiently consolidated to withstand the scouring action that 

 recurred later when this spot was no longer sheltered from the 

 sweep of the tides. That the calcareous cementing-matter has been 

 mainly derived from this source is indicated by the differential state 

 of preservation of the shells — those which were originally composed 

 of calcite being beautifully preserved, while the species with shells 

 of aragonite are scantily represented, and are generally in the 

 form of casts. The classical researches of Dr. H. C. Sorby into the 

 relative durability of fossils of aragonite and calcite, and their 

 application by later workers to explain the selective preservation of 

 organisms in limestone and other rocks, are too well-known to need 

 more than a passing reference. 



The presence of similar indurated calcareous masses in the ' Sponge- 

 Gravels ' of the ' Faringdon Pit ' may here be recalled ; but there is 

 this difference in the conditions, that at Faringdon the concretions 

 are developed in a matrix which is still crowded with organisms 

 and abundantly calcareous, while at Shenley the fossils are confined 

 to the blocks. 



General Characters of the Fossils. 



With regard to the palajontological aspect of the fossils, brachiopods 

 are by far the most numerous, both in species and in individuals ; 

 next in abundance come the pectens, some of which are highly 



