C H A 



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G H A 



the seeds of some plants, it is some- 

 times coloured : the lemon and 

 orange afford examples of the 

 chalaza. 



CHALCE'DONY. See Cakedony. 



CHALK, (balk, Germ., the craie 

 blanche of the French.) A white 

 earthy limestone, composed of lime 

 and carbonic acid; a variety of 

 carbonate of lime. According to 

 M. Berthier, the chalk of Meudon, 

 when the sand disseminated in it 

 was separated by washing, con- 

 tained in 100 parts, carbonate of 

 lime 98, magnesia and a little iron 

 1, alumina 1. It has an earthy 

 fracture, is meagre to the touch, 

 and adheres to the tongue ; it is 

 dull, opaque, soft, and light ; its 

 specific gravity being from 2.30 to 

 2.80. It contains an inconsidera- 

 ble proportion of silex and iron. 

 The harder varieties of this sub- 

 stance were formerly used for 

 building, and, when protected from 

 the influence of the atmosphere by 

 a thin casing of limestone or flint, 

 proved very durable. The ruins 

 of the Priory of St. Pancras, near 

 Lewes, which have stood nearly 

 800 years, prove this. Dr Mantell. 

 The rock commonly known as 

 chalk preserves its peculiar mineral 

 character throughout a considerable 

 area in Europe, but it is rarely of 

 such thickness as in many parts of 

 the south east of England, where 

 horizontally stratified masses about 

 one thousand feet thick are compo- 

 sed of it. In proportion as we 

 depart from the great central deposit 

 of Europe, we find the chalk greatly 

 varying in its texture; in some 

 parts becoming oolitic. 



Yarious conjectures have been 

 offered respecting the probable 

 origin of chalk, and the mode 'of its 

 formation. Patrin supposed that 

 it was the production of three 

 different causes. 1. Animal earth, 

 proceeding from the decomposition 

 of organic bodies. 2. Calcareous 



lava, ejected by submarine volcanos. 

 3. Detritus of calcareous moun- 

 tains. Delametheric imagined it 

 to have been deposited by water in 

 a state of great agitation. Dr. 

 Mantell observes that it may have 

 been precipitated from water hold- 

 ing lime in solution, from which 

 an excess of carbonic acid was 

 expelled. Mr. Bakewell says, 

 " according to Eerrare, streams of 

 liquid chalk, or chalk in the state 

 of mud, were ejected from the mud 

 volcano of Macaluba, in Sicily, in 

 1777. If then we allow submarine 

 aqueous eruption of calcareous 

 matter, and siliceous solutions from 

 thermal waters, to have been poured 

 into a deep ancient ocean, we shall 

 have all the circumstances required 

 to form thick beds of chalk, inter- 

 spersed with nodules of flint. Mr. 

 Lonsdale, on examining some pieces 

 of chalk from various parts, found 

 what appear to the eye simply 

 white grains, were, in fact, well 

 preserved fossils. Erom each pound 

 in weight of chalk, he obtained 

 about 1,000 of these. 

 CHALK FOBMATION. This term is 

 applied in the nomenclature of 

 geology to a group of deposits 

 very dissimilar in their lithological 

 compositions, but agreeing in the 

 character of the organic remains 

 which they contain, and referrible 

 to the same epoch of formation, or 

 series of strata, of great depth, 

 which are spread over a large 

 portion of the eastern and south- 

 eastern counties of England, 

 northern Erance, Germany, Den- 

 mark, Sweden, European and 

 Asiatic Russia, and the United 

 States of North America. It 

 consists of strata that have been 

 accumulated in the depths of a sea 

 of vast extent, and affords a striking 

 illustration of the character of 

 oceanic deposits. Scarcely a trace 

 of chalk can be found in Scotland 

 or Wales, but it occurs in Ireland 

 M 



