Scientific Lectures. J27 



ftir and ■water upon tlie exposed portions of the earth's crust, convei't- 

 ing the sihca into chxj, with carhonates of lime, magnesia, and soda 

 through the action of the carbonic acid of the atmosphere. The 

 soda carried by rains to the sea, decomposes the lime salts, forming 

 carbonate of lime and sea salt. The process is still going on, thongh 

 more slowly, from the small amount of carbonic acid in the air, and 

 causing the decay in the hearts of granite rocks. We have thus 

 explained the generation of silica or quartz of clay and of limestones, 

 the principle elements of sedimentary rocks. Ever}- clod of clay 

 represents granite rocks decomposed, and an amount of limestone 

 and sea salt, formed from the waters of the ocean. In this way the 

 air was freed from carbonic acid, and fitted for the support of animal 

 life. Besides this, the vegetation removed large portions of carbonic 

 acid, replacing it by oxygen, and the formation of limestone directly 

 diverted still greater amounts of carbonic acid, whose presence must 

 have rendered the early atmosphere unfit for the higher forms of life. 

 The presence of carbonic acid in the early atmosphere serves to 

 explain the higher temperature then prevailing, which permitted the 

 gi'owth of tropical plants within polar circles. We know that a 

 portion of carbonic acid, such as then existed in- the air, while it 

 would not prevent the passage of the sun's rays would impede the 

 radiation of obscure heat from the earth's surface, and thus tend to 

 keep up a summer temperature. The efiect of this carbonic acid 

 would be like the glass of an orchard-house in preventing the escape 

 of heat. Tims carbonic acid exerted also an important part in many 

 other chemical processes then active at the earth's surface. Besides 

 deposits formed by chemical processes, mechanical operations were 

 forming at the earth's surface a gi'eat amount of sandy and clayey 

 rocks, which make up the bulk of the stratified forms. Although 

 the interior of the earth has been regarded as solid, it is notwith- 

 standing doubtless intensely heated, and thus is explained the increase 

 of temperature as we go below the surface. The cooling of this 

 center, once rapid, is now xery slow indeed from the thickness 

 of the overlying sediment. The efiect of this heat upon the 

 deeply buried sediment has been to crystallize them, and convert 

 them into metamorphic rocks. To this class belongs granite, 

 once looked upon as a primitive rock. We have now evidence 

 that granite is in all cases a secondary rock, derived from sediments 

 crystallized through the agency of water and heat. In the quartz of 

 granite are ofcen found small cavities, partly filled with water, which 



