104 THE WONDERS OF GEOLOGY. Lect. I. 



beneath great pressure, are totally different from those 

 effected by fire on the surface, under the ordinary weight 

 of the atmosphere. A familiar example will serve to illus- 

 trate my meaning. Chalk consists of lime combined with 

 carbonic acid ; and as, for agricultural and other economical 

 purposes, it is desirable to have the lime in its pure state, 

 the chalk, or limestone, is exposed to a great heat, in kilns 

 erected in the open air, until all the carbonic acid gas is 

 dissipated, and the stone is said to be burnt into quicklime. 

 In the specimens before us the same substance is seen in the 

 state both of chalk and lime. It may readily be conceived, 

 that if this operation were conducted beneath such a degree 

 of pressure as would prevent the escape of the gaseous ele- 

 ments, the formation of quicklime would not take place ; the 

 chalk would be fused, and the carbonic acid, released from 

 its present relation with the calcareous particles, would enter 

 into other combinations, and the mass when cooled, be wholly 

 different from the product of the lime- kilns, formed by the 

 same agency in the open air. Experiments have proved 

 that this opinion is correct. Sir James Hall exposed 

 pounded chalk to intense heat, under great pressure, and it 

 was fused, not into lime, but into crystalline marble : even 

 the shells inclosed in the chalk underwent the same trans- 

 mutation, yet preserved their forms.* That analogous 

 changes have been effected by natural operations we have 

 abundant proof ; but in this stage of our inquiry it is only 

 necessary to remark, that where ancient streams of lava have 

 traversed chalk, the latter invariably possesses a crystal- 

 line structure. We shall hereafter find, in accordance with 

 the philosophical theory of Dr. Hutton, that all the ancient 



* From a table drawn up with due caution by Sir James Hall, it is 

 proved that under a depth of the sea, not exceeding one-third of a 

 mile, chalk would be converted into a crystalline limestone, and that 

 at a depth of little more than a mile it would be in a state of fusion, 

 provided there were no refrigerating causes in action. 



