16 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.! 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 
80,000 feet, has been mainly accumulated under 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. Itis by this simple method of studying present 
actions that nearly all difficult questions in geology may be solved. 
The leading principle of the geologist is to interpret the past by 
1 See The Autobiography of the Earth, p. 223. 
