292 Geology. — On the Crust of the Earth. 



study of Geology. This is one of its first and most important 

 lessons, and the solution to the inquiry will be found to be the 

 key to similar phenomena, in situations still more extraordinary. 

 To trace these rounded pebbles to their native rocks — and on 

 the dry lands they are sometimes found, hundreds of miles from 

 their parent beds — they must be compared with other pebbles, 

 which are generally to be found strewed through countries, to 

 the original masses. Here a slight knowledge of mineralogy, 

 one of the branches of geology, is necessary. The next question 

 the young geologist asks himself is, whether the whole substance 

 of the planet is one solid mass of rocks resembling those he finds 

 near the surface. If he has hitherto observed no mineral differ- 

 ence in the rocks he has examined, and if the territory under 

 examination furnishes several members of the geological series 

 of beds superimposed upon each other, he will find some indica- 

 tions of the presence of those members either in the mural es- 

 carpments on the sea coast, in the ravines inland which have been 

 worn by the action of rivers, in valleys or depressions, or in 

 the fissures which have been caused by any natural agents. In 

 such situations he will often find the mineral structure of the 

 rocks corresponding on the opposite sides, where the continuity 

 of the surface has been interrupted, the same beds presenting 

 themselves on each side. When he is fully satisfied that there 

 are various mineral beds lying beneath the arable soil on the 

 surface containing rolled pebbles, he will be still more anxious 

 to learn the nature of all the beds lying beneath those he has 

 examined. And thus it is by travelling and practical investigation, 

 and by books and the conversation of learned men, that the 

 young geologist at length comes to the knowledge, that the earth 

 is not a mass of rocks and clays and sands, accumulated without 

 order and design; but that a portion of the superficial part of the 

 planet, now called the crust, is composed of a series of rocks, 

 clays and sands, differing from each other in very material cir- 

 cumstances, and of which the respective members are, at very 

 great geographical distances, found in a constant relation to each 

 other, in the general geological series of beds. This geological 

 series has been described with gr^at fidelity, and will be the 

 subject of our next Essay. 



By the crust of the earth, we understand that portion of it of 

 which we have a practical knowledge. There are certain rocks 



