LESSON FROM A LUMP OF CHALK. 167 



continued to do down to the invasion of the locomotive and 

 the transformations of civilization. 



I can not undertake to convey to your comprehension, in 

 a few lines, any adequate conception of the aspects and char- 

 acters of these long extinct beings. If we speak only of 

 mammals, we can say that they generally differed widely 

 often grotesquely from any forms now living. It was then 

 near the beginning of mammalian development on the earth. 

 Still those creatures presented unmistakable resemblances to 

 modern mammals, in all fundamental respects. If there were 

 no elephants, there were the Brontotherium and Dinoceras and 

 Tinoceras, and especially in the Old World, the Dinotfierium, 

 which seemed like uncouth and undeveloped pachyderms trying 

 to become proboscidians. But the Brontotherium had no tusks, 

 no trunk, no elephantine molars. The "comprehensive" char- 

 acter of these and other early mammals was the most inter- 

 esting fact ; but I reserve more particular mention for a later 

 opportunity. 



XXIX. LESSON FROM A. I^TJNIP OR CHALK. 



MESOZOIC ROCKS AND FOSSILS. 



THIS white lump, soft enough to be cut with a knife, effer- 

 vesces very briskly when any strong acid is applied to it. 

 Even strong vinegar causes the formation of a multitude of 

 small bubbles. Effervescence is caused by the escape of some 

 gas. Almost always, the gas is carbonic acid, or as we now 

 say, carbon dioxide. Chalk is a compound of this and cal- 

 cium. The latter is familiar in the form of lime. Carbonic 

 acid is feeble, and when the strong acid is applied to the 

 chalk, it drives off the carbonic acid, and takes possession of 

 the calcium for itself, forming a different compound. The 

 carbonic acid when freed from combination, resumes its 

 gaseous form. It therefore swells up, and mixed with the 

 water of the strong acid, produces the bubbles which consti- 

 tute effervescence. Chalk, however, has essentially the same 

 constitution as limestone and marls. 



