IO88 THE MICROSCOPE IN GEOLOGICAL INVESTIGATION 



the proportion which not only the distinctly organic remains bear to 

 the amorphous residuum, but also the different kinds of the former 

 bear to each other ; and this is quite what might be anticipated when 

 we remember how one or another tribe of animals predominates 

 in the several parts of a large area ; but it may be fairly concluded, 

 from what has been already stated of the amorphous component of 

 the Globigerina-rnud, that the amorphous constituent of chalk like- 

 wise is the disintegrated residuum of foraminiferal shells, or at any 

 rate of some small calcareous organism. But, further, the Globigerina- 

 mud now in process of formation is in some places literally crowded 

 with sponges having a complete silicious skeleton ; and some of them 

 bear such an extraordinarily close resemblance, alike in structure 



FIG. 812. Microscopic organisms (chiefly f ora mini f era) in chalk from Meudon, 

 seen partly as opaque, and partly as transparent objects. 



and in external form, to the Ventriculites which are well known as 

 chalk fossils, as to leave no reasonable doubt that these also were 

 silicious sponges living on the bottom of the cretaceous sea. Finally 

 (as was first pointed out by Dr. Sorby) the coccoliths and cocco- 

 spheres at present found on the sea-bottom are often to be discovered 

 by the microscopic examination of chalk. 1 All these correspondences 

 show that the formation of chalk took place under conditions 

 essentially similar to those under which the deposit of Globigerina- 

 mud is being formed over the Atlantic sea-bed at the present time. 

 In examining chalk or other similar mixed aggregations, whose 



1 'On the Organic Origin of the so-called " Crystalloids " of Chalk ' in Ann. Nat. 

 Hist. ser. iii. vol. viii. 1861, pp. 193-200. Murray and Renard, Deep Sea Deposits 

 (Challenger Reports), p. 257. 



