THE MESOZOIC AGES. 227 



deposit of light-coloured and usually soft calcareous 

 matter attains in some places to the enormous thick- 

 ness of 1,000 feet. Nor is it limited in extent. 

 According to Lyell, its European distribution is from 

 Ireland to the Crimea, a distance of 1,140 geo- 

 graphical miles; and from the south of France to 

 Sweden, a distance of 840 geographical miles. Simi- 

 lar rocks, though not in all cases of the precise nature 

 of chalk, occur extensively in Asia and in Africa, and 

 also in North and South America. 



But what is chalk ? It was, though one of the most 

 familiar, one of the most inscrutable of rocks, until 

 the microscope revealed its structure. The softer 

 varieties, gently grated or kneaded down in water, or 

 the harder varieties cut in thin slices, show a con- 

 geries of microscopic chambered shells belonging to 

 the humble and simple group of Protozoa. These 

 shells and their fragments constitute the material of 

 the ordinary chalk. With these are numerous spicules 

 of sponges and silicious cell- walls of the minute one- 

 celled plants . called Diatoms. Further, the flinty 

 matter of these organisms has by the law of molecular 

 attraction been collected into concretions, which are 

 the flints of the chalk. Such a rock is necessarily 

 oceanic ; but more than this, it is abyssal. Laborious 

 dredging has shown that similar matter is now being 

 formed only in the deep bed of the ocean, whither no 

 sand or mud is drifted from the land, and where the 

 countless hosts of microscopic shell-bearing protozoa 

 continually drop their little skeletons on the bottom, 



