552 Notices of Memoir >i — L. J. Spencer — On Zircon. 



most abundant phosphatic fossils of these places is the ammonite, 

 usually fragmentary, which has habitually been named Amm, hiphx, 

 but belongs in almost every case to one or another of several allied 

 species of Lower Cretaceous Olcostephani. Most of the lamelli- 

 branchs can likewise be best matched by Lower Cretaceous forms; 

 and there are good grounds for suspecting that many of the Saurian 

 and fish remains from the above-mentioned places and from the 

 Faringdon 'Sponge-Gravels' which have been classed as Jurassic 

 are true Lower Cretaceous forms. 



It is acknowledged that the presence of transported pebbles of 

 older rocks in the deposits at Upware, Potton, and Faringdon 

 renders the occurrence of derivative fossils at these places more 

 probable than in the case of the Speeton and Lincolnshire ' coprolite- 

 beds ' ; and in the collections examined a few specimens were 

 noticed that seem to have been washed from older rocks. But the 

 writer believes that these instances are exceptional, and he urges 

 that no fossil should be set down as derivative unless the evidence 

 is conclusive, as much confusion has arisen through the unquestioning- 

 adoption of the hypothesis of derivation. 



While there is still much to be leax'nt as to the physical conditions 

 requisite for the concretion of phosphatic nodules and for their 

 segregation into bands, it seems clear that an important determinative 

 was the existence of submarine currents occasionally impinging upon 

 the sea-floor with sufficient sti'ength to sweep away the matrix in 

 which the nodules had been formed, so that there was a gradual 

 accumulation of the partially eroded nodular residues. Such residues, 

 though of inconsiderable thickness, may represent a long period of 

 submarine conditions. The term ' aggregate deposits ' has been 

 suggested by J. F. Blake for beds of this character. 



IL — On the different Modifications of Zircon. By 

 L. J. Spencer, M.A., F.G.S.' 



SOME very irregularly developed crystals of zircon from the 

 gem - washings of the Balangoda district in Ceylon were 

 found to have characters differing widely from those of zircons 

 of more common occurrence. Although of low specific gravity 

 (4'0), they are not increased in density when strongly ignited, as 

 are many zircons of specific gravity below 4-7. They further differ 

 from ordinary zircon in their very feeble, or absence of, birefringence. 

 The crystals are dark brown in colour and almost opaque, but after 

 ignition they are bright green and quite transparent. 



While some of the crystals consist wholly of zircon of this type, 

 others contain an intergrowth of a second kind, which may be 

 present in greater or less amount. The latter has a higher specific 

 gravity, and increases in density when ignited ; it is optically biaxial 

 with very strong birefringence. A section cut perpendicular to the 



1 Abstract of paper read before the British Association, Cambridg-e, Section C 

 (Geology), August, 1904. 



