Professor W, J. Sollas — Derived Limestones. 249 



denudation of a limestone and tlie transportation of calcareous 

 sediment may be accomplished not only by solution but by the 

 mechanical action of fresh-water. 



On examining under a microscope the suspended matter which 

 floats in the river Cam above Cambridge, and which may readily 

 be separated by filtration through a muslin net, the observer who 

 looks upon them for the first time will be surprised^ to find 

 associated with a number of minute living organisms certain bodies 

 which resemble in the closest manner the coccoliths of the Chalk. 

 The appearance of these is so fresh that it would not be an altogether 

 inexcusable blunder to regard them as native to the stream, and 

 though it might be objected that fresh-water coccoliths are things 

 liitherto unheard of, yet an answer to this might be found in the 

 fact that on removing the calcareous part of the organism with 

 dilute acid a soft granular film remains behind, which has not only 

 a very organic appearance, but readily stains with magenta and some 

 ■other aniline dyes. 



On pushing the enquiry further, however, it will be found that 

 the residual film is not affected by such stains as are selective in 

 their effects ; those which only react upon protoplasm have no effect 

 upon it. Further, the coccoliths give no signs of life ; they cannot 

 be made to grow nor to subdivide by fission. 



In the old days when the coprolite pits around Cambridge were 

 being worked it was not uncommon to come across streams of 

 chalky water flowing away from the washing tanks. An examina- 

 tion of this revealed all the forms of coccoliths which were to be 

 found floating in the Cam, but in this case there could be no 

 question as to their origin, they were derived from the chalk marl. 



Considering, then, that no independent evidence can be cited for 

 the existence of fresh-water coccoliths, that the minute bodies 

 floating in the Cam present no signs of life, and that the river drains 

 a country largely composed of chalk rocks, from which, as we have 

 proof, coccoliths may be derived, the presumption is altogether in 

 favour of regarding the coccoliths of the Cam as mechanical sedi- 

 ments now in process of being carried to the sea. 



I have said that these sediments may be "considerable." A good 

 deal depends on what is meant by considerable, but I think the 

 qualification may be justified by the following example. On 

 visiting the gravel-pits at Barnwell occasional white lenticular beds 

 may be observed intercalated amid the finer sands of the deposit. 

 These are composed of carbonate of lime, and a microscopic examina- 

 tion proves them to consist of minute organisms and debris of 

 organisms, which have been derived from the chalk. Foraminifera, 

 whole and broken, of which Globigerina is one of the commoner 

 forms, shell prisms, and coccoliths are obvious. Thus we learn that 

 not only may a river transport calcareous sediment, even when con- 

 stituted of such minute bodies as coccoliths, just as it might an 

 insoluble clay, but further, that under favourable circumstances 

 it may deposit this material and thus form beds of limestone, which 

 are not immediately but only indirectly of organic origin. 



