418 PROCEEDINGS OF THE GEOLOGICAL SOCIET5T. [May 14, 



and appearance, which is not the case. Further, two contiguous 

 pieces of chalk, which for the most part would be of the same 

 density and free from hard extraneous particles, would not scratch 

 but polish each other. The author suggests an organic origin for 

 these markings, and regards them as the traces of Corals. 



Discussion. 



Prof. Duncan, though differing entirely from the author, had 

 been struck with the reasons which he had for his surmises. He 

 exhibited some specimens of recent West-Indian corals, which offered 

 at first sight much resemblance to the character shown in the Chalk. 

 It was to be observed that no reef-building corals are occupants of 

 the deep seas, in which there is little doubt the Chalk was deposited. 



Mr. H. "Woodward thought that in one or two instances a pseudo- 

 morph of a Siplwnia might be discerned. He could not, however, 

 accept the fibrous structure as organic. It appeared to him to re- 

 sult from the infilling of cavities in the Chalk. 



Mr. Whitaker read a note from Mr. Judd, who held that " these 

 specimens were not due to any organic origin. The quasi- crystal- 

 line structures known as ' cone-in-cone,' or ' beef,' seemed to him 

 closely related ; and in the Yorkshire Chalk an incipient crystalliza- 

 tion of this kind had been set up, and the fact of its existence sub- 

 sequently developed by the solvent action of water, without which 

 it might have remained latent in the body of the Cbalk. In one 

 instance the fibrous structure of a portion of the shell of an Inoce- 

 ramus was continuous with that of the matrix." Mr. Whitaker was 

 glad that the author had called attention to the subject, and thought 

 that all would go with him in doubting that the structure was due 

 to slickensides. He mentioned the presence of similar striae in the 

 Chalk of Surrey. Another observer in Yorkshire had called atten- 

 tion to the greater abundance of the markings in the neighbourhood 

 of fissures than in the solid Chalk. 



Mr. Evans mentioned the occurrence of similar structure in the 

 Chalk of Hertfordshire. The portions of the rock in which it oc- 

 curs are usually harder than the surrounding rock ; and in some 

 cases the structure might be seen to pass into that of the ordinary 

 chalk. He considered it to be mainly due to chemical causes. 



Mr. Prestwich suggested the desirability of analysis to determine 

 whether there was any difference in the chemical constitution of the 

 fibrous and non-fibrous chalk. 



Mr. Forbes, referring to the possibility of this structure being due 

 to crystallization, thought its occurrence in the harder parts of the 

 chalk in favour of this view ; chalk, being a nearly pure carbonate of 

 lime, might crystallize either as calcite or aragonite. The specimens 

 in question showed no trace of the peculiar cleavage of calcite, but 

 had a strongly developed fibrous structure resembling aragonite ; 

 and as this mineral is the most instable form of crystalline carbonate 

 of lime, it would account for its subsequent amorphous condition. 

 He thought, as these specimens were more commonly found near 



