364 Dr. T. Sterry Hunt's Lecture— 



and thirdly, to the formation of carbonate of lime or limestone ; and 

 in these three elements you have, as it were, the alphabet of all our 

 stratified or water-formed sedimentary rocks. Sand (broken up 

 quartz), clay, and lime, in various proportions, either in their simple 

 mechanical mixture or, secondly, combined by chemical action, make 

 up the whole of the rocks with which we have to deal, — of course 

 including iron and some other metals which intervene, to a very 

 secondary degree however. 



Thus I have endeavoured, so far, to show you how these processes 

 naturally going on, give rise to the elements which make up these 

 stratified and sedimentary rocks. It belongs to physical geology to 

 explain the breaking down of these sediments and their mechanical 

 distribution, and the subsequent origin of the different varieties of 

 rocks ; and we can only indicate the general chemical law which 

 has thus presided. 



But here you see not only the origin of these three great classes 

 of minerals which make up the rocks, but you see the origin of the 

 saltness of the sea. That is a question which has very much 

 puzzled and perplexed many who have written upon the subject, — 

 whence the sea derived its salt. It has been said by some writers 

 that the source of the salt was to be found in the land, because salt 

 was recognized as an element in the rocks ; and some even went so 

 far as to contend that the origin of this salt was to be found in 

 igneous action. They imagined in the centre of the earth a source 

 from which all its materials were extruded, and rock-salt among 

 them ; but a very careful series of investigations upon the composi- 

 tion of ancient waters, the mineral waters, which are nothing less 

 than the fossil sea-waters locked up in the ancient rocks, have 

 shown that there has been a slow progressive change in the consti- 

 tution of the ocean, and that by means of the action of carbonate of 

 soda, derived from the decay of the solid rocks, there have been 

 separated the whole of the carbonates of lime, which make up the 

 calcareous strata — the marbles and the various limestones which we 

 find on the earth's surface. It is not unusual to say that these 

 limestones are the result of organic action, because we find them to 

 a great extent made up of shells, of corals, and of the remains of 

 calcareous animals. That is very true, but these animals can only 

 appropriate the carbonate of lime which they find ready formed at 

 their hands ; and that carbonate of lime has been formed from the 

 salts of the sea by reason of this peculiar decomposition which I 

 have just now explained. 



But this takes us a step farther ; for if the rocks are decomposed 

 by virtue of the carbonic acid of the air, there has been a great 

 separation of carbonic acid from the atmosphere. You will recollect 

 that we supposed that at first the whole of the carbon which has 

 since gone into the solid crust of the earth was diffused in our 

 atmosphere, thus giving a composition very different from that 

 which we have at the present time. Now, we all know that this 

 carbonic acid in large proportion is unfavourable to the development 

 of the higher forms of life ; and we see, therefore, in this slow 



