Such, however, are after all only secondary incentives to the study of the rocks. As a mental exercise, geology certainly yields to none of the other sciences, for it addresses itself at once to the reasoning powers and to the imagination, and may thus be made a source both of intellectual training and of delightful recreation. Of none of the sciences is it so easy to get a general smatter, yet none is so difficult thoroughly to master, for geology embraces all the sciences. In so wide a field, the student will therefore find ample room to expatiate. In beginning the study, he may perhaps think it, as Milton pictured the other paths of learning, "laborious, indeed, at the first ascent; but else so smooth, so green, so full of goodly prospects and melodious sounds on every side, that the harp of Orpheus was not more charming." If time and taste disincline him to travel over the whole of the broad field, there are delightful nooks to which he may betake himself, replete with objects of beauty and interest, where he may spend his leisure, and by so doing not merely delight himself, but enlarge the bounds of human knowledge. No part of the domain can be too obscure or remote to reward his attention; no object too trifling or insignificant: for the march of science, though a stately one, proceeds not by strides, but by steps often toilsome and slow; and she stands mainly indebted for her progress not to the genius of a few gigantic intellects, but to the united efforts of many hundred labourers, each working quietly in his own limited sphere.