Prof. Ansted on the Zoological condition of Chalk Flints, ^c. 247 



received upon a mass of semifluid siliceous matter, ^\'ithout the 

 existence of which it has hitherto been tliought impossible to ac- 

 count for the pheenomena. The microscopic structure is in all 

 these cases precisely the same, and the evidence is therefore con- 

 clusive. 



And finally, the composition of the chalk itself and the natm'e 

 of its organic remains, requiring as these do the lapse of a long 

 period of time, may be considered to strengthen the probability 

 of the organic origin of the siliceous part of the formation. There 

 is, we knovvby experience, a tendency in different substances, when 

 distributed irregularly, to accmuulate round certain points of at- 

 traction, and by the process of segregation separate themselves 

 even from others with which they are in contact, and this is still 

 more the case when any particles of the same substance as that 

 about to be grouped are present in any part of the whole mass. 

 Sponges are known to contain siliceous spicula which also are 

 constantly seen in chalk flints, and thus we have additional 

 grounds for supposing that the layers of flint were formed upon 

 spongy bodies as their centre. 



It is only necessary now in conclusion to put into general 

 language the view which results from the considerations I have 

 offered with reference to the internal structure and external 

 appearance of chalk flints, and their relations v\ath the fossils of 

 the chalk and with the chalk itself. 



I suppose that for a long period the gradual deposit of chalk 

 may have gone on undistui-bed, the material being derived partly 

 from animals living in the water at the time, and partly, perhaps 

 chiefly, from the degradation of calcareous and coralline rocks at 

 a distance, the silt being conveyed by marine currents and evenly 

 deposited. In this way I presume that the formation of the lower 

 and middle chalk may be fairly explained. At the termination 

 of these deposits I imagine that there succeeded a period of 

 tranquillity, the cretaceous mud ceasing to be deposited, and the 

 bottom of the sea becoming hard and fit for the habitation of in- 

 numerable sponges, which lived and flourished there. In these 

 we see, as I believe, the elements of the first layer of flint in the 

 chalk, and I suppose that after they had continued to exist for a 

 certain time, there occurred the commencement of a series of sub- 

 terraneous distm-bances in some part of the bed of the ocean of 

 the nature of undulations, elevating the bed of the sea in some 

 parts and depressing it in others. These disturbances I also sup- 

 pose to have been accompanied by the eruption of a considerable 

 quantity of hot water holding silica in solution*. The elevation 



* The boiling springs of the Geyser in Iceland contain 31-38 grs. of silex 

 per gallon of water, and even the mineral waters of Bath contain 20 grs. in 

 ten pints and a half. 



