1 6 EXTINCT MONSTERS. 



mouth of that great river, the Amazon, the sea is discoloured by 

 fine sediment. 



There is another kind of rock frequently met with, the building 

 up of which cannot be explained in the way we have pointed 

 out ; and that is limestone. This rock has not been deposited as 

 a sediment, like clays and sandstones, but geologists have good 

 reasons for believing that it has been gradually formed in the 

 deeper and clearer parts of oceans by the slow accumulation of 

 marine shells, corals, and other creatures, whose bodies are 

 partly composed of carbonate of lime. This seems incredible 

 at first, but the proofs are quite convincing. 1 As Professor 

 Huxley well remarked, there is as good, evidence that chalk has 

 been built up by the accumulation of minute shells as that the 

 Pyramids were built by the ancient Egyptians. 



The science of geology reveals the startling fact that all the 

 great series of the stratified rocks, whose united thickness is over 

 N^8o,ooo feet, has been mainly accumulated rnider water, either by 

 the action of those powerful geological agents rain and rivers 

 or through the agency of myriads of tiny marine animals. When 

 we have grasped this idea, we have learned our first, and, perhaps, 

 most useful lesson in geology. 



Now let us apply what has been above explained to the question 

 immediately before us. We want to know how the skeletons of 

 animals living on land came to be buried up under water, among 

 the stratified rocks that are to be seen all over our country, and 

 most of which were made under the sea. 



We can answer this question by going to Nature herself, in 



order to find out what is actually going on at the present time, by 



inquiring into the habits of land animals, their surroundings, and 



the accidents to which they are liable at sundry times and in 



divers manners. It is by this simple method of studying present 



actions that nearly all difficult questions in geology may be solved. 



V The leading principle of the geologist is to interpret the past by 



1 See The Autobiography of the Earth, p. 223. 



