LESSON FROM A LUMP OF CHALK. 169 



of correlation of organic structure with environment. Faunas ^ 

 change as the physical surroundings change ; but if the physi- 

 cal surroundings remain changeless, the faunas remain change- 

 less. Down in the deep sea, with a constant temperature of 

 freezing water ; with perpetual absence of the stimulus of 

 light ; without motion ; without change of chemical condi- 

 tions with almost absolute changelessness for ages, why should 

 organisms change ? They are now suited to the environment ; 

 they could not change without becoming unsuited to the en- 

 vironment. The forms from the Age of Chalk have survived 

 because a deeper principle than that of succession of faunas 

 has been dominant. It is the principle of correlation of en- ^ 

 vironment and organism. The animal must be adapted to its 

 surroundings. Nearly all the populations which have lived 

 dwelt on land or in comparatively shallow water, where 

 environment was undergoing progressive change ; hence suc- 

 cession of faunas. A few deep sea species have dwelt where 

 change of physical conditions is almost unknown; hence a 

 nearly changeless fauna. Thus a piece of chalk reveals a deep 

 and important principle. 



The position of the chalk-beds in the series of geological 

 formations is nearly at the top of the Cretaceous System. The 

 System, besides the beds of chalk, contains strata of sand and 

 clay. One variety of sand is green, and in New Jersey, oppo- 

 site Philadelphia, it is dug extensively for fertilizing soils, 

 since, as you will infer, it is not a purely silicious sand, but 

 contains a large percentage of potash. The Cretaceous strata 

 extend along the belt parallel to the Atlantic and Gulf coasts, j 

 into Mexico ; but from Maryland to Georgia, the Atlantic belt 

 is mostly covered and concealed by the Tertiary beds. From 

 middle Georgia, a broad belt extends into eastern Mississippi, 

 and thence north to the Ohio river near Cairo. West of this, 

 the Cretaceous strata are concealed by Tertiary and Mississippi 

 alluvium, as far as Little Rock. Near here an exposed belt 

 begins which widens extensively toward the southwest, through 

 Texas. Remember that the place of the Tertiary strata is 



always between the Cretaceous and the ocean. The Cretaceous 



15 



