SPECIAL GEOLOaT. 



prevailed, up to a very recent period, as to the origin of 

 chalk and flint was this: It was supposed that thermal 



FIG. 268 Junction of vertical and contorted Chalk Strata at Ballard Head, Dorset- 

 shire. The cliff is 352 feet high ; the down, 584 feet. 



waters, charged with calcareous and siliceous matter, were 

 poured into the ocean, and that, on mingling with the colder 

 element, the calcareous and siliceous substances were pre- 

 cipitated in a solid state, and, separating by the laws of 

 chemical affinity, deposited the chalk and flint. 



Many distinguished observers, however, had ascribed these 

 substances to a different cause. A century ago, Linnaeus 

 declared his conviction that calcareous strata were of animal 

 origin, and the recent observations of microscopic observers 

 have tended to confirm the opinion of this great naturalist. 

 Chalk, when reduced to powder, and appearing like mere 

 grains of dust to the eye, consists, in fact, of well-preserved 

 fossils of the beautiful forms depicted in figures 186 and 187 ; 

 but these specimens are really gigantic, when compared with 

 the smaller extinct forms of life which are contained within 

 them ; the chambers of these foraminifera being filled with 

 thousands of well-preserved infusoria, whose skeletons 

 abound in the cretaceous rocks. "We have only to brush a 

 piece of chalk in water, dry the particles thus obtained, and 

 examine the residuum with the microscope, and we shall 

 find it consisting of minute shells, corals, or portions of 

 these. 



As many other rocks are found on microscopic examina- 



