bS THE APPLICATION OF THE MICROSCOPE 



examining this sediment, that it is formed, in the main, by the 

 accumulation and disintegration of the shells of Globigerina, the 

 shells, fresh, whole, and living in the surface-layer of the deposit, 

 and in the lower layers dead, and gradually crumbling down by 

 the decomposition of their organic cement, and by the pressure of 

 the layers above." 



Now, the resemblance which this Globigerina-mud, when dried, 

 bears to chalk is so close, as to suggest a similar origin of the 

 latter, and this is at once confirmed by microscopic observation. 

 Many samples of it consist, in great part, of the minuter kinds 

 of Foraminifera, especially GIobigeri?ice, whose shells are embedded 

 in a mass of apparently amorphous particles, many of which 

 nevertheless present indications of being the worn fragments of 

 similar shells or of larger calcareous organisms ; whilst in other 

 places, the chief part is made up of the shells of Entomostracous 

 Crustaceans. And, further, the Globigerina-mud now in process 

 of formation is in some places crowded with sponges, having a 

 siliceous skeleton, and some of these bear such an extraordinary 

 resemblance in structure and in internal form to the Ventriculites, 

 which are well known as Chalk fossils, as to leave no reasonable 

 doubt that these also lived as sponges on the bottom of the 

 Cretaceous sea. Other sponges, also, are found in the Globigerina- 

 mud, the structure of whose horny skeleton corresponds so closely 

 with the sponge-tissues, which can be recognised in sections of 

 nodular flints^ as to make it clear, when taken in connection with 

 the correspondence of external form, that such flints are really 

 fossilised sponges, the silicifying material having been furnished 

 by the solution of the skeletons of the siliceous sponges, or of 

 Diatoms or Radiolaria. 



There are other deposits of less extent and importance than 

 the great chalk formation, which, like it, are composed of micro- 

 scopic organisms, chiefly Foraminifera, and the presence of 

 animals of this group may be recognised by the assistance of the 

 microscope, in sections of calcareous rocks of various dates, whose 

 chief materials seem to have been derived from Corals, Encrinite 

 stems, or the shells of Molluscs, as in the Crag formation of the 

 Eastern coast of England. 



Many parts of the Oolitic formation have an almost identical 



