102 



KNOWLEDGE 



[June 1, 1892. 



stated that in some rare cases they may even rise to as 

 much as ninety per cent. 



We have now, therefoi-e, to add to our definition of Chalk 

 that it is largely composed of the shells of the minute animals 

 known as forams, together with those of other allied crea- 

 tures, and so may accordingly speak of it as a limestone 

 which is evidently to a large extent of organic origin. More- 

 over, as these forams are more or less closely allied to species 

 inhabiting the ocean at the present day, we should be 

 justified from this evidence alone in regarding the Chalk 

 as a formation of marine origin. This origin is, however, 

 equally well proved by the larger fossils, such as shells of 

 sea-urchins, scallops, oysters, etc., commonly occurring in 

 the Chalk ; while, in addition to this, the extreme purity 

 and thickness of the formation would of itself be sufficient 

 to demonstrate that the Chalk is the result of long- 

 continued deposition on the bottom of the sea. 



Thus much for the composition of our lump of Chalk as 

 examined in the laboratory, and we now turn, as all 

 geologists worthy of the name should, to its occurrence 

 in the field. If we look at one of the tall Chalk cliffs of 

 our southern coasts, as in the neighbourhood of Dover, we 

 shall be first of all struck with the extreme homogeneity 

 and purity of the whole formation from top to bottom, 

 through a thickness which in this neighbourhood is close 

 upon 1000 feet, and in Norfolk is more than 1100 feet. 

 This similarity of composition throughout such a vast 

 thickness is totally unlike what we are accustomed to 

 observe in other rock-cliff's (although there is some 

 approach to it in the blue mountain-limestone of Derby- 

 shire), where we generally find alternating bands composed 

 of rocks differing both in colour and structure from one 

 another, and we are thereby led at once to conclude that 

 there must be something very peculiar connected with the 

 deposition of the Chalk. How was it that in the old sea 

 there were not only no currents bringing loads of sand or 

 clay to alternate with the pure white limestone, but, above 

 all, that there was not a tinge of colouring matter to stain 

 the virgin purity of the newly-formmg Chalk during those 

 ages and ages of time, while drifted logs and fruits occur 

 but rarely ? 



A closer inspection of a large thickness of Chalk will, 

 however, reveal the fact that there is not a complete 

 similarity in the nature of the rock throughout the entire 

 formation. Thus, whereas in places where nearly the 

 whole formation is displayed we find throughout the 

 uppermost 100 feet layers and nodules of Hint are 

 thickly distributed throughout the mass, generally form- 

 ing more or less well-marked lines which indicate 

 the original planes of the deposition of the rock, as we jiass 

 to a lower level the proportion of tliese flints becomes 

 gradually less, till, after we have passed downwards 

 through some 130 feet, they finally disappear, and are 

 wanting throughout the whole of the lower part of the 

 series. Moreover, in this lower Chalk, or Chalk without 

 flints, we shall find, as we pass downwards, a gradual 

 tendency to lose the pure white colour of the upper Chalk, 

 and to assume a buff or greyish tint, while in the very 

 lowest beds we shall not fail to notice the appearance of a 

 number of small grains of a greenish-coloured mineral. 

 If, again, we try to dissolve this lower Chalk in acid we 

 shall find that as we descend in the series there is an ever 

 increasing quantity of an insoluble remnant, which would 

 be shown by analysis to be of the nature of clay. Both 

 these circumstances point to the conclusion that at the 

 time the lower Chalk was laid down the conditions were 

 by no means so well adapted for the deposition of a pure 

 carbonate of lime as was the case in the later time of the 

 upper Chalk with tiiuts. What these conditions were we 



shall consider subsequently, but we have now to direct 

 our attention to the area over which the white Chalk 

 extends. 



In the north-west the furthest limits to which the white 

 Chalk extended are found near Belfast, where, as we have 

 said, the rock has been converted into a hard limestone by 

 the action of heat. Although we do not again meet with 

 Chalk till we reach the east and south of England, where 

 it forms large poitions of our coast from Dorsetshire to 

 Yorkshire, yet it is probable that the Chalk sea embraced 

 the foot of the Welsh mountains, which formed an 

 archipelago. From England the white Chalk may be 

 traced without any alteration in its character through the 

 north of France, the south of Belgium, the eastern part of 

 the Netherlands, and thence through Westphalia, Hanover, 

 and Galicia, into Poland and Russia, where it reaches on 

 the one side to the foot of the Urals, and on the other to the 

 Crimea ; moreover, to the northward it occupies a con- 

 siderable portion of Denmark and the southern extremity of 

 Sweden. Although the white Chalk is now only distributed 

 over the surface of this region in larger or smaller patches, 

 being sometimes covered up by newer (Tertiary) deposits, 

 and in other places totally wanting, there is evidence that 

 it once extended continuously over the whole. Moreover, 

 the absence of any traces of the white Chalk in the regions 

 to the west and north of those mentioned, indicates that 

 the present limits of the Chalk in those directions mark 

 approximately the boundaries of this cretaceous sea : this 

 sea being probably cut off' from free communication 

 with the Atlantic by a barrier connecting western France 

 with Cornwall and Ireland, and by another joining 

 Scotland with Scandinavia. 



The above area includes the whole of the white Chalk ; 

 but when we trace this Chalk southwards into Bohemia 

 and Saxony we find that it has undergone a very 

 remarkable change. Thus, although it contains the same 

 fossils as to the northward, the rock itself, instead of being 

 the pure white limestone to which we have been accustomed, 

 consists of a series of massive sandstones about as unlike 

 Chalk as anything well could be. It is probable, indeed, 

 that these cretaceous sandstones, as we may call them, 

 were formed in a gulf on the southern coast of the white 

 Chalk sea, which was unfavourable to the deposition of 

 Chalk itself ; and as these sandstones were undoubtedly 

 deposited at the same time as the pure Chalk, we thereby 

 learn the very important geological lesson that similarity 

 or dissimilarity in the mineralogical structure of a rock is 

 a matter of very minor import indeed. We may illustrate 

 this by reference to architecture. Thus, a Gothic church 

 may be built either of sandstone, limestone, marble, or, 

 for the matter of that, brick ; but it will still be (exclusive 

 of course of our so-called modern Gothic) absolutely 

 characteristic of one particular period of European archi- 

 tecture. This Gothic style will be distinguished by certain 

 peculiarities in the structure of its arches and pillars, as 

 well as by the ornaments with which they are embellished. 

 •Just so in geology we have a Chalk or cretaceous style, in 

 which, although the rock itself may be either Chalk or 

 sandstone, or limestone, or slate, yet its architectural 

 details — that is to say, its fossils— will be the same, not 

 only throughout Europe, but within certain limitations of 

 variation, over the whole world. This is one of the 

 important lessons to be learnt by a comprehensive study of 

 our white Chalk. 



The second great lesson taught by the white Chalk is, 

 however, of perhaps still more importance. We have seen 

 that the white Chalk was deposited in a sea cut oft' from 

 free communication with the Atlantic to the west and 

 north ; and the range of the Ardennes which formed its 



