2 THE CHAIN OF LIFE. 



Geology as a practical science consists of three leading parts. 

 The first and most elementary of these is the study of the dif- 

 ferent kinds of rocks which enter into the composition of those 

 parts of the earth which are accessible to us, and which we are 

 in the habit of calling the crust of the earth. This is the sub- 

 ject of Lithoiogy, which is based on the knowledge of minerals, 

 and has recently become a much more precise department of 

 science than heretofore, owing to the successful employment of 

 the microscope in the investigation of the minute structure and 

 composition of rocks. The second is the study of the arrange- 

 ment of the materials of the earth on the large scale, as beds, 

 veins, and irregular masses ; and inasmuch as the greater part 

 of the rocks known to us in the earth's crust are arranged in 

 beds or strata, this department may be named Stratigraphy. 

 A more general name sometimes employed is that of Petro- 

 graphy. The third division of geology relates to the remains 

 of animals and plants buried in the rocks of the earth, and 

 which have lived at the time when those rocks were in 

 process of formation. These fossil remains introduce us 

 to the history of life on the earth, and constitute the subject 

 of Palceontoiogy. 



It is plain that in considering what may be learned as to 

 epochs in the history of life we are chiefly concerned with the 

 last of these divisions. The second may also be important as 

 a means of determining the relative ages of the fossils. With 

 the first we have comparatively little to do. 



Previous to observation and inquiry, we might suppose that 

 the kinds of animals and plants which now inhabit the earth 

 are those which have always peopled it ; but a very little study 

 of fossils suffices to convince us that vast numbers of creatures 

 once inhabitants of this world have become extinct, and can 

 be known to us only by their remains buried in the earth. 

 When we place this in connection with stratigraphical facts, 

 we further find that these extinct species have succeeded each 

 other at different times, so as to constitute successive dynasties 



