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\ HISTORV OF CHALK 



) i iwari! \. Martin, i G.S 



I itm.it from pttgt 



CRB1 M ROUS STR VTA Oi.PEI ii i HA1 K. 



Si i II aci limitations of sedimentary matter that 

 ied under the ( In I lem, 



by the affinities of their respective fossilil 



, cannol be consider d lly, of ver) 



ancienl date in the world's history. It is true they were 

 accumulated m in; me long before the 



advenl the existing species of animals, and 



therefore ages before thi man upon the 



earth. As compared, however, with the age of the rucks 

 formed i 1 imbrian times, the creta- 



ceous rocks are bu date. The • ilde 



sedimentar; formations of which we have any certain 

 information, have been in existence so long, and have 



many welding forces brought to bear upon 



them, that, while we ari ibl to isolate thi cretaceous 



system, extending at mosl to a thickness 



iul 4, tooft. > the top, yet the 



characters in the more ancient rocks arc so uniform, 



and so crystalline, and the fossils havi I ra altered, 



that we have no means of grouping thosi strata, 

 excepl bj pis m so large a thick- 



strata as 50,0001't. in the case of the Laurentian 

 in Am 1 i jo.oooft. in that of the Cambrian, 



includin ian. It will thus be seen that the 



various "systems" arc very far from being of 

 uniform thickness, and as a matter of fact, h 

 they decrease in the thickness (if their contents as 

 approach is made In the present day. 



The group of strata known as cretaceous is met in 

 the sequence of geological history at the top of the 



ic systems. Its true position will lie seen the 

 more easily by a glance at the following li-i of 

 formations : — 



Tertiary 



Pleistocene- 

 Pliocene. 



01 



Miocene. 



Cainozoic 



t lligocene. 





Ei icene. 





CRETACEI Us. 



Sei 1 mdary 



Ne mian. 



or 



Jurassic. 



Mes 



Lias. 





Trias. 





' Permian. 





Carl oniferous. 



Primary 



1 ir 



I levonian. 

 Silurian. 



Palaeo 1 lie 



' Irdovician. 

 Cambrian. 





Pre-Cambrian 





(Archaean). 



With the progress of geological discovery, it has 



been found that these d 



visions, although convenient 



for sj stematic purp 



hard and fast limit. "P constantly 



being discovered, which unite in thi 



hand to the strata above, 11 

 ■ .1I1. 1 in those beli iw . 

 It must not he imagined that the familiar rock known 



as Chalk is the sole constituent of thi 1 irma 



t !• 111. In fact, in Great Britain this system contains, in 

 aboul equal proportions, calcareous, argillaceous, ami 

 arenaceous strata. 



The Chalk, although so well-marked a sub-division, 

 is thus of no greater geological importance than any 

 1 Hi' iihers, lint from familiarity, it has come to lie- 

 regarded as the most prominent member of the 

 in. whilst from the beauty ami perfection of its 

 fossils, it will ever remain to the collector one of the 

 tracti geological strata. 



It will have been noticed in the arrangement given 

 above, thai «■■ have adopted the classification that 

 places Neocomian beds mi the list as a separate 

 formation. The beds classified under this heading 

 are those known as the Wealden lulls and the 

 Lower Greensand. They are still classed by some 

 geologists as I 1 retaceous. The fresh-water 



Wealden beds are regarded, in their upper part, as 

 having been built up in an estuarine sea, whilst the 

 Lower Greensand, at least in some parts, was con- 

 temporaneously forming in an oceanic area. There 

 is no doubt, however, that the latter formation is 

 divided from the Chalk in point of time, by a far 

 greater interval than had formerly been imagined. 

 The general opinion is that it should he separated 

 from the Cretaceous, and classed as a distinct system 



under the title ol Neoc hi : 1 In- title also including 



the Purbecks and the Wealden. There is no definite 

 sequence between the three, hut there is another 

 formation, away from the then disturbed area 

 of the South-east of England, which repre- 

 sents to us in a most fortunate manner, the 

 regular succession of life during this period, from the 

 close of the well-defined Kimmeridge Clay to the 

 early days of the Gault. This is the Speeton Clay of 

 the Yorkshire coast. It there attains a thickness of 

 marly 300ft. After examination, it has yielded 

 evidence of being divisible into well-marked /.ones. 

 These /ones represent phases of life at different 

 periods, which correspond closely with the succes- 

 sion of Portland Meds, Purbecks, Wealden, Lower 

 Greensand, and early Gault, and may be connected 

 with them as follows : — 

 Zone ol ''■■>' w«'«lV«l«=Gault. 



,, B. semi-eana!iculatiis=ljoviex Greensand. 

 ,, B. jaculum— Wealden (apparently I. 

 ,, />'. lateralis^ Portland and Purbeck Beds. 

 ( To be continued. ) 



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