but slightly inclined, it will be broader; the breadth of such a line (which may be coloured to taste) always varying with the thickness of the stratum and the angle which it makes with the horizon. In a district where faults and curvatures along with trap-rocks abound, the mapping becomes more complex, but the principle remains the same--a curved stratum on the ground making a similarly curved band on the map, and a fault or dislocation of a set of beds producing, in the same way, a corresponding break in the lines traced. In short, a geological map should be as far as possible a transcript of the surface rocks of a country. The beginner should avoid, however, attempting too much; it will be enough for him at first to have mastered the leading features of the geology of his district; the details cannot be shown save on a map of a large scale, and are better transferred to his note-book. The use of such mapping is to enable us to gain a correct knowledge of the geological structure of a country, and of the relation of rocks to each other as regards age, origin, &c. Bacon tells us that "writing makes an exact man;" we may say with equal truth that mapping makes an exact geologist. It is sometimes easy enough to obtain a notion of the general character of a district by taking a few rambles across it; but we can never know it thoroughly until we have mapped it. And this is done not as mere dry routine, or by a series of hard uninteresting rules. In reading off the geological structure of a country, we ascertain its history during many thousand ages long prior to that of man. We become, as it were, interpreters of hieroglyphics, and historians of long-perished dynasties. Those who have had experience of field-geology, know how vain it is to attempt to compress into a page or two the results of years, and that a few vague general directions are about the utmost that can be attempted. The practice of the science cannot be taught in books, far less in prefaces, neither can it be learned from them. And so I once more repeat the advice: Get away to the fields. Seek to decipher the geological records for yourself, and look with your own eyes into the long series of ages whose annals lie inscribed among the rocks. If you can secure the co-operation of a few companions, so much the better. Half-a-dozen hammers zealously at work in a richly fossiliferous stratum will soon pile up a tolerable collection of its treasures. But whether singly or in company, use your eyes and your hammer, and even though in the end you should never become a geologist, you will in the meantime gain health and vigour, and a clearness of observation, that will stand you in good stead through life. CONTENTS. CHAPTER I. PAGE Scene near Colinton in midsummer--A grey travelled Boulder--Its aspect and contents--Its story of the past, 1 CHAPTER II.