114 Notice of Dr. MantelVs Medals of Creation. 



whether in the bed of an ocean or of a lake or estuary — by the 

 action of the sea or of rivers — of lakes or of running streams — by 

 the effects of icebergs or glaciers — by slow processes through 

 long periods of time, or by sudden inundations or deluges, or by 

 the agency of volcanoes and earthquakes." 



Dr. Mantell remarks that the entire series of rocks within the 

 scope of human examination, is estimated at a thickness of from 

 fifteen to twenty miles, reckoning from the summit of the high- 

 est mountains to the greatest depth hitherto penetrated ; and as 

 this thickness scarcely equals 4 ^o^h of the diameter of our globe, 

 it is familiarly termed the earth's crust. 



We are pleased to observe that the allowed thickness of the 

 earth's known crust is here taken at about twice what it was 

 formerly estimated, but including the knowledge which we gain 

 from the inclination of the strata and from the ejection of ancient 

 and modern igneous masses, there can be no doubt that we un- 

 derstand its constitution to a much greater depth, even hundreds 

 of miles. 



The author divides the fossiliferous rocks into 



1. " The tertiary ; comprising all the deposits from the allu- 

 vial drifts to the chalk. 



2. " The secondary ; from the chalk to the old red, or Devo- 

 nian system, inclusive. 



3. " The Silurian ; from below the Devonian to the upper 

 part of the Cambrian system, in which all traces of organic re- 

 mains disappear." 



The second chapter contains the classification of the stratified 

 rocks upon, 1, their mineral structure ; 2, their order of superpo- 

 sition ; and 3, the nature of the organic remains they contain. 



The great and subordinate divisions correspond with those in 

 general use among English geologists : we cite only the leading 

 members. 



I. Drift, ancient and modern. 



II. The Tertiary system, with its Pliocene, Miocene and 

 Eocene. 



III. The Chalk, with seven subdivisions. 



IV. The Wealden, with four subdivisions. 



y. The Oolite, with five greater and sixteen lesser subdi- 

 visions. 



VI. The Lias, with four subdivisions. 



