CHALK. 395 



CHALK. 



The Chalk is one of the best known rocks ; It forms some of the 

 marked features in our island, it is developed over a large area, and 

 from its nature is more easily identified than any other formation. 

 The name was used geologically by jNIartin Lister in 1684. 



The Chalk may be described as a white limestone, for it is 

 almost wholly composed of carbonate of lime, but the lower beds 

 usually are less pure than the upper, and sometimes become sandy 

 and at others sufficiently argillaceous to form a marl, Avhile here 

 and there thin seams of laminated marl occur in higher beds of 

 the Chalk. The layers of Chalk are generally much fissured 

 and fractured, they sometimes attain a thickness of 3 or 4 feet, 

 and are usually wedge-shaped masses, tapering in different direc- 

 tions. These divisional lines seem in some instances to be due 

 to a kind of horizontal jointing rather than to any direct process of 

 deposition.^ Throughout the greater part of the Chalk there occur 

 nodules of black and grey flint, usually in bands that coincide with 

 the stratification of the rock. Thin tabular veins of flint are also 

 met with in places, and these appear not only along the lines 

 of bedding, but often obliquely, and even vertically in reference 

 to it. The lowest beds of Chalk are usually destitute of flints ; but 

 an analysis of Chalk Marl from near Farnham showed over 21 per 

 cent, of silica and alumina.^ 



The following analyses of Chalk were made by David Forbes : •* — 



White Chalk, Grey Chalk, 



Shoreham, Sussex. Folkestone. 



Carbonate of lime 98'40 94'09 



,, ,, magnesia 0'o8 0-31 



Insoluble rock debris (silica) rio 3-61 



Phosphoric acid ) 



Alumina and loss 0*42 / trace 



Chloride of sodium 1-29 



Water o 70 



These analyses show the proportions of carbonate of lime in the upper and 

 lower beds, but probably in selected specimens ; were an extensive mass of Chalk 

 analysed, the amount of silicate of alumina might prove to be larger. 



Here and there erratic boulders have been met with in the Chalk, such as 

 granite, greenstone, quartz, quartzite, sandstone, schist and coal ;^ but on the whole 

 it is remarkably free from sand or pebbles, except at its western extremity near 

 the junction with the Greensand below. It contains many nodules and sometimes 

 crystals of iron-pyrites, locally called ' thunderbolts,' also manganese-ore in specks 

 and dendritic markings. 



1 T. Webster, T. G. S. ii. 174 ; see also P. F. Kendall, G. Mag. 1884, p. 551. 



2 J. T. Way and J. M. Paine, Journ. R. Agric. Soc. xii. 544. 



^ Q. J. xxvii. (p. xhx) ; Whitaker, Geol. London Basin, p. 15. See also 

 Pennmg and Jukes-Browne, Geol. Cambridge. The chloride of sodium may 

 perhaps be derived from sea-spray. 



* Dixon's Geology of Sussex, new ed. by T. R. Jones, p. 129 ; Jones, Trans. 

 Hertfordshire Nat. Hist. Soc. iii. 147. 



