424: 



MESOZOIC ERA— AGE OF REPTILES. 



muddy shores, as also sun-cracks and rain-prints, would be found as on 

 other shores. Now, although in the strata associated with salt organic 

 remains are rare, shore-marks of all kinds are common. Thus de- 

 posits of gypsum and salt may be taken as evidence of dry climate. 



-r- 



Section 2. — Jurassic Period. 



This is the culminating period of the Mesozoic era and Eeptilian 

 age. In it all the characteristics of this age reach their highest de- 

 velopment. We must discuss this somewhat more fully than the last. 

 The strata belonging to this period are magnificently developed in 

 the Jura Mountains, and hence the name Jurassic. These mountains 

 are an admirable illustration of the manner in which ridges and valleys 



are formed by the fold- 

 ing of strata (Fig. 

 626) ; they also abound 

 in fossils of this pe- 

 riod. 



English geologists 

 call the period Oolite 

 (egg-stone) on account 

 of the abundant oc- 

 currence in that coun- 

 try of a peculiar limestone composed often wholly of small, rounded 

 grains like the roe of a fish. They divide the whole period into three 

 epochs, viz. : 1. Lias; 2. Oolite proper ; 3. Wealden* They also sub- 

 divide the Oolite proper into Lower, Middle, and Upper Oolite, sepa- 



Pig. 626.— Section of the Jura Mountains. 



Lower 

 Oolite. 



Middle 

 Oolite. 



Upper 

 Oolite. 



London 

 Chalk. Clay. 



Lias. 



Oxford Clay. 

 Fig. 627. 



Kimmeridge 

 Clay. 



Gault. 



rated by intervening Oxford and Kimmeridge clays. All these divis- 

 ions and subdivisions are well shown in the following section passing 

 from London westward. This section is interesting not only as exhib- 

 iting all the divisions and subdivisions of the Oolitic period, but also 

 as showing their conformity among themselves and with the overlying 

 chalk, and the unconformity of these with the overlying Tertiary. It 

 also shows how parallel ridges and intervening hollows are formed by 

 the outcroppings of a series of strata alternately hard and soft. 



Origin of Oolitic Limestones. — These are composed of rounded grains 

 with concentric structure. We have already seen, page 156, that on 



The Wealden is now often put in the Cretaceous. 



