on the Organic Composition of Chalk and Chalk Marl. 305 



The reader being thus put in possession of the general scope 

 of the work, I now proceed to exhibit in full the conclusions 

 to which the author has been led (as indicated under the head 

 of No. 1 1), to which 1 shall subjoin further extracts taken from 

 different portions of the Memoir, for the purpose of general 

 illustration. 



Conclusions, 



1. Many, and probably all. White Chalk Rocks are the pro- 

 duce of microscopic coral-animalcules, which are mostly quite 

 invisible to the naked eye, possessing calcareous shells of J^ 

 to -^^-Q line in magnitude, and of v^hich much more than one 

 million are well preserved in each cubic inch, that is, much 

 more than ten millions in one pound of chalk*. 



2. The Chalk Marls of the Mediterranean Basin are the 

 produce of microscopic Infusoria possessing siliceous 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 animalcules, but it proceeds from a disintegration 

 of the assembled microscopic organisms into much minuter 

 inorganic calcareous particles ; the reunion of which into re- 

 gular, elliptical, granular laminae, is caused by a peculiar cry- 

 stalloid 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 deve- 

 loped to the greatest extent. 



4. The compact limestone rocks also which bound the Nile 

 in the whole of Upper Egypt and extend far into the Sahara 

 or Desert, being neither white nor of a staining quality, as 

 well as the West Asiatic compact 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 Libya from Syene to the 



* It is to be understood that I speak only of such Polythalamia as are 

 well preserved, wholly disregarding their fragments. Of the well-preserved 

 there are contained in one fourth part of a cubic line, or in one twelfth of 

 a grain of chalk, frequently 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 

 1,382,400 in each cubic inch ; and hence in one pound of chalk the num- 

 ber far exceeds ten millions. 



The larger Polythalamia and Bryozoa of the chalk are best obtained from 

 the sediment produced by brushing the chalk under water ; the entirely 

 microscopic forms remain long suspended in tvater. 

 Ann. ^ Mag. N. Hist. Vol. vii. X 



