86 



XX. EHRKNBERG'S CONCLUSIONS ON THE ORGANIC COMPOSITION 



OP CHALK AND CHALK MARL.* 



1. MANY, and probably all, White Chalk Rocks are the produce of 

 microscopic coral animalcules, which are mostly quite invisible to the 

 naked eye, possessing calcareous shells of -^ to -^^ line in magnitude, 

 and of which much more than one million are well preserved in 

 each cubic inch, that is, much more than ten millions in one pound of 

 chalk, f 



2. The Chalk Marls of the Mediterranean Basin are the produce of 

 microscopic Infusoria, possessing silicious shells or cases, mostly quite 

 invisible to the naked eye, intermingled with a small proportion of the 

 calcareous animalcules of the chalk. 



3. The peculiar state of aggregation in White Chalk does not arise 

 from a precipitate of lime previously held in solution in the water of 

 the sea, nor is it the result of the accumulation of the small animal- 

 cules, but it proceeds from a disintegration of the assembled microsco- 

 pic organisms into much minuter organic calcareous particles ; the re- 

 union of which into regular, elliptical, granular laminse, is caused by a 

 peculiar crystalloid process, which may be compared to crystallization, 

 but is of a coarser nature, and essentially different from it. The best 

 writing chalk is that in which this process has been developed to the 

 greatest extent. 



4. The compact limestone rocks, which bound the Nile in the whole 

 of Upper Egypt, and extend far into the Sahara or Desert, being nei- 

 ther white nor of a staining quality, as well as the West Asiatic com- 

 pact limestone rocks in the north of Arabia, are, in the mass, composed 

 of the coral animalcules of the European Chalk. This affords a new 

 insight into the ancient history of the formation of Lybia from Syene 



* Extracted from Mr. Weaver's View of Ehrenberg's Observations on this subject 

 in the Annals of Natural History for June, 1841, p. 305. 



f It is to be understood that I speak only of such Polythalamia as are well pre- 

 served, wholly disregarding their fragments. Of the well preserved there are con- 

 tained in one-fourth part of a cubic line, or in one-twelfth of a grain of chalk, fre- 

 quently 150 to 200 in number, equal to 600 800 in each cubic line, or 1800 2400 

 in each grain, and from 1,036,000 to 1,382,000 in each cubic inch ; and hence in 

 one pound of chalk the number far exceeds ten millions. 



The larger Polythalamia and Bryozoa of the Chalk are best obtained from the sedi- 

 ment produced by brushing the chalk under water ; the entirely microscopic forms 

 remain long suspended in water. 



