464 NOTES. 



strata laid open by the excavations of their currents; but his most instructive 

 studies will be found, when he has arrived far inland at the mountains, where they 

 take their rise. Here he will find that nature has revealed the structure of the 

 globe on the grandest scale ; here the marks of ancient revolutions will be found 

 imprinted in characters not to be mistaken, and the truth both of facts and 

 theories, before known only by description, will at once be impressed on his mind. 

 By researches of this kind, extended over considerable tracts of countries, so as to 

 embrace all the great series of geological formations, and by careful study and 

 comparison of all the phenomena presented to his view, both as regards the 

 mineral structure of the globe, the forms of organized bodies peculiar to each 

 species of rocks, and the physical changes now taking place on the earth's surface, 

 the student wifl at length become a practical geologist, and be enabled by his own 

 observations to improve and advance the science he has been studying. But the 

 course which has here been pointed out, although essential to a practical and 

 thorough knowledge of the subject, can only be pursued by few; and a general idea 

 of its most important facts, and the practical consequences arising from them, is 

 of comparatively easy attainment. The great principles of geology have been most 

 ably brought together in various publications ; and where only a general knowledge 

 is required, geological maps and sections may be made in some measure to supply 

 the place of travelling and observation. A few words then on these important 

 documents, which are the medium of expressing some of the most important 

 practical results of the labours of geologists in the field, may not be misplaced. A 

 map which combines with the geographical and physical features of a country, 

 a view of its internal structure, supposes all wood and vegetation to be absent, 

 and that every species of superficial soil and covering removed, so that the 

 actual rocks and strata which compose the solid crust of the globe beneath shall be 

 perfectly exposed and laid open to our view. The space occupied at the surface by 

 these rocks and strata is then distinctly shown by different tints of colour, in the 

 same manner as territorial divisions are indicated on ordinary geographical maps. 

 But although we thus obtain a perfect view of the surface distribution of the solid 

 materials of the globe, it is evidently essential to know in what manner they are 

 arranged below, and what relations they bear to each other in the internal parts of 

 the elobe. This object is accomplished by means of geological sections, the nature 

 of which will acquire but little explanation. A geological section supposes, that on 

 any given line the internal structure of the earth is laid open in the direction of a 

 vertical plane, as in our section between Barley Moor and Grange Mill in Derby- 

 shire. It therefore merely represents, although generally on a much more 

 extended plane, the same thing which we see in many artificial excavations, and 

 which nature herself exhibits to our view in cliffs and precipices. Geological 

 sections are indeed merely a combination of sections of this kind, in which they 

 bear the same relation as the map of a large country would do, to the smaller 

 plans and sketches from which it was compiled, especially connected with the 

 Mechanics of Fluids. Such a map is that of Messrs. J. and C. WALKER. 



It appears that the present annual value of the mineral produce of Great 

 Britain, may be estimated at somewhere about 20,000,000?. sterling; independent 

 of any subsequent process of manufacture, and not including the cost of carriage 

 on coal. Burr's " STUDY OF GEOLOGY," London, 1836. 



