H. C. Sargent— The Crick In Her. 163 



20 feet above sea-level, upon whicli a landing was effected by the 

 officers of a British marine survey ship shortly after the eruption, 

 when the mud still indicated a temperature of 148 deg. Fahr. a few 

 feet below the surface." 



V. A jN^OTE on the CARBOJSriFEROTJS LiMESTONE OF THE 



Ckich Inliee. 

 By H. C. Sargent, F.G.S. 



ICHA]N"CED a few weeks ago to see, for the first time, a short 

 paper by Mr. J. Wilfrid Jackson, F.G.S., in the Geological 

 Magazine (Dec. Y, Vol. V, p. 266) on "Mottled Foraminiferous 

 Limestone in West and North Lancashire". In that paper 

 Mr. Jackson thus describes specimens of mottled limestone from 

 Silverdale : "The matrix is light-coloured and fine-grained, with 

 irregularly shaped patches (? nodules) of a fine-grained daik-coloured 

 limestone. The patches have no definite shape, being mostly very 

 uneven in size, some of them being long cylindrical bodies with 

 irregular contours." 



Mr. Jackson writes with special reference to a paper by Messrs. 

 Barnes & Holroyd, in which they describe limestone of the same 

 character from North Derbyshire,^ and similar mottled beds may 

 also be seen in the quarries near Crich in the southern part of the 

 county. 



Messrs. Barnes & Holroyd, in the paper referred to, show, as 

 I think, conclusively that the difference in colour between the dark 

 and light portions is due to a larger proportion of carbonaceous matter 

 in the former than in the latter. They go on, however, to say that 

 this "is caused by the absence in the light part of the remains of 

 foraminiferal life, while in the dark we have abundant life remains, 

 and these in a most perfect state of preservation". On this point 

 Mr. Jackson states as follows : "In the Silverdale specimens, so far 

 as I have made out, foraminifera are almost as numerous in the light 

 portion of the rock as in the dark," - and this observation agrees with 

 ray own examination of thin slices prepared from the Crich rock. 



Messrs. Barnes & Holroyd are further of opinion "that the dark 

 and light are not the same limestone, but that the dark has been 

 removed from its original place of deposition and intermingled and 

 cemented into the lighter matrix ".^ They consider that this process 

 has been effected by volcanic activity. " Given that these outbursts 

 of volcanic activity were preceded by the usual earth shocks and sea 

 disturbances, and the neighbouring presence of calcareous beds of 

 dark foraminiferal mud, we have all the conditions necessary for the 

 turning up from the sea bottom, and consequent removal to another 

 area of the dark parts, together with the redeposition and consolidation 

 to produce the peculiar mottlings." * Whereupon Mr. Jackson 

 naturally inquires whether these effects of volcanic activity in 



^ " On the Mottled Carboniferous Limestone of Derbyshire " : Trans. Manch. 

 Geol. Soc, vol. xxvi, p. 561. 



^ Loc. cit. ^ Loc. cit. * Ibid. 



