82 Reports and Proceedings. 



geological specimens, neatly arranged in a polished deal box, prepared 

 expressly to illustrate the Geological Primer (price 10s. M.) are sup- 

 plied by Mr. James K. Gregory, 15, Eussell Street, Covent Garden, 

 London, or may be had of Messrs. Macmillan, the publishers. 



Geological Society of London, — L — December 3, 1873. — Joseph 

 Prestwich, Esq., F.R.S., Vice-President, in the Chair. The following 

 communications were read : — 



1. " Notes on the Structure sometimes developed in Chalk." By 

 H. George Pordham, Esq., F.G.S. 



After referring to Mr. Mortimer's paper on the same subject (see 

 Quart. Journ. Geol. Soc. vol. xxix. p. 417), the author stated that in 

 a pit near Ashwell the " Lower Chalk without flints " exhibits a bed 

 of a concretionary nature, the concretions in which are marked 

 nearly all over with lines. The lines are found only on the con- 

 cretions and in their immediate neighbourhood. The fossils in the 

 bed are invariably crushed, as if by pressure. The author believes 

 that the stri^ are due to an incipient crystallization arising from the 

 formation of the concretions ; and in support of this view he adduced 

 a specimen of iron pyrites from the chalk of Beachy Head, attached 

 to which is a small portion of very hard striated chalk, and suggested 

 that the crystallization of the pyrites had induced a crystallization 

 in the chalk. He considers, however, that in some places an almost 

 identical structure may be due to slickensides, but only in very 

 broken and faulted beds. 



Discussion. — Mr. Seeley observed that the structure was famihar to all. If it 

 were due to crystallization, whether incipient or otherwise, he wished to know to 

 what combination with lime for a base the form of the crystals was due. He 

 thought that a certain amount of phosphate of lime was present in the concretions, 

 but was absent in similar specimens in the Upper Chalk, so that it appeared as if 

 the same cause could not apply in both. The strias were not, he thought, due to 

 slipping or to organic growth, but might arise from some alteration in the character 

 of the chalk. 



Mr. Evans observed that the striae appeared to be due to two causes — crystalli- 

 zation, whether incipient or destroyed, and slickensides. He thought that in the 

 Ashwell specimens much was owing to the hard nodules resisting pressure better 

 than the surrounding chalk, which, in being condensed, passed over their surfaces 

 and produced a kind of slickensides. The same appeared to have been the case 

 with the nodule of pyrites. 



Mr. Forbes remarked that an inspection of the specimens on the table convinced 

 him that in several instances the structure was due to slickensides, but that in 

 others traces of a very different structure were visible, which he imagined was due 

 to crystallization, the carbonate of lime having most probably assumed the form of 

 Aragonite, which, owing to its instability, had lost its crystalline lustre, and 

 assumed a meal}^ or chalky appearance. 



Mr. Judd thought that the contrast between the two papers communicated to the 

 Society was striking. This was mainly due to the diffei-ence in the Chalk of 

 Yorkshire and that of the south of England. The abundance of this structure in 

 Yorkshire might well have caused Mr. Mortimer to connect it with some organic 

 origin for the formation of the Chalk. Its rarity in the south might have led Mr. 

 Fordham to assign another and a chemical cause. He thought that it prevailed 

 most in those parts of the Chalk through which water most readily passed, and 

 considered that in some cases the crystallization had been that of Aragonite, in 



