a J. L. LOBLEY — CRETACEOUS BOCKS OF ENGLAND. 



scenery, the agriculture, and the general character of a district as 

 the Chalk, forming as it does those softly-rounded Downs, treeless 

 for the most part, and covered with a springy velvety turf, which 

 give to England her great sheep-grounds of the southern counties. 

 ISo formation affords so striking an example of the importance of 

 the study of the geological structure of a district as a preparation 

 for the proper consideration of its Natural History. An illustration 

 of this is at hand. In the beautiful park of Cassiohury, adjoining 

 "Watford, beeches famed for their beauty abound ; while these trees 

 are not found to flourish nearer London. Geology supplies the 

 reason, for it tells us that though at Cassiohury we have the Chalk, 

 with a thin covering of superficial deposits, the London Clay under- 

 lies the country to the south, and supports elms and oaks rather 

 than beeches, which grow most luxuriantly over the Chalk. Many 

 other facts might be stated to show that T^aturaHsts generally, 

 whether Botanists, Entomologists, or Meteorologists, will be mate- 

 rially assisted in their studies by making themselves acquainted 

 wdth the geological structure of the district which is to become the 

 field of their investigations. 



The origin of the Chalk must now engage our attention. The 

 aqueous or sedimentary rocks have all been formed by the accu- 

 mulation of sediment deposited at the bottom of water. But the 

 deposition of the sediment which has produced these rocks has been 

 of a threefold character. Sediment may accumulate at the bottom 

 of water from (1) simple mechanical action, or the sinking of 

 particles of solid matter previously held in suspension by the water ; 

 from (2) chemical action, or the precipitation of solid matter pre- 

 viously held in solution by the water; or (3) from organic action, 

 or the formation of solid matter in the form of shell, bone, wood, 

 etc., by animals or plants. The Chalk has been produced by the 

 third of these processes, and is therefore, as geologists say, an 

 organically formed rock. 



One of the lowest Classes of the Animal Kingdom, called the 

 Foraminifera or hole-beaiing animals, comprises creatures so small 

 that to see them the aid of the microscope is required. These 

 minute animals, although themselves consisting merely of a jeUy- 

 like substance called sarcode, have the power of extracting from the 

 sea- water carbonate of Hme, and of secreting this in a solid form, and 

 so encasing themselves with a thin shell. This shell is perforated by 

 minute canals, through which fine threads of sarcode are protruded, 

 to expand and form other little shells, and so continue growtli. 

 Sometimes the compound animal takes the form of a spiral, whik' 

 sometimes it is elongated, and various modifications of the spiral 

 and of the linear forms constitute the several genera and species 

 which have been described. Of these microscopic shells the Chalk 

 consists. What an expanded view of time, and of the operations of 

 Nature, must be given to any one wlio huinis for the first time that 

 the great South Coast (!lifis, which im])rt'ss(>d our Koman invaders 

 and gave the name Albion to our island ; thaf the great masses of 

 white rock forming the Downs and Wolds of England, and com- 



