ries an impressive lesson as to the unity prevailing amid all the diversity of Nature, besides affording the hope that we may at some time discover the origin of life, since it has already opened the way to an explanation of the origin of the existing forms of life; while the grand outcome of geological study is that it brings vividly before the mind the immensity of time, enabling us to realize that time is only less than eternity. It also teaches us that our earth has had a history, that our own race has had a high antiquity; and thus the contemplation of past geological ages, reckoned by millions of years, the fact that our earth is coeval with the sun in age—all these considerations tend to immeasurably expand our mental horizon, and thus to react in a way to broaden the mind.
Geology is also the complement of biology. As soon as one has mastered the rudiments of botany and zoology, and of the distribution of life-forms in space, the range of his thoughts should be extended to take in the orderly succession of life in past ages, and the evolution of modern specialized plants and animals from the earlier, generalized types. No liberally educated person can, then, afford to ignore the study, and it seems to us that it should be taken up for the following, among many other considerations:
1. Our first reason is that geology throws light on the origin of our earth and of the solar system in general; the facts and speculations which culminated in the modern nebular hypothesis give some idea of the steps by which our planet assumed its present form and became adapted for the maintenance of life.
2. After the earth cooled down and assumed its present shape and size; in some way unknown to us, monads and bacteria, together with infusoria, one-celled plants and animals, began to exist, and geology hints that the period when all this became possible may have been the early Laurentian, or at least at the dawn of what, for a better name, we call archsean time.
3. We now feel quite sure that the diversity of life of the Cambrian period must have been in some way the result of great changes in the physical geography of that time, and correlated with the inequalities of the sea-bottom, with regions of shallows and of abysses, with landlocked areas, islands, and incipient continents, rising from submarine plateaus bearing mountain-chains. Geology describes the birth of continents, the rise of mountain-chains, and discusses the results of the action of heat in transforming the physical features of our globe, and thus, in part at least, explains the origin of volcanoes, the causes of earthquakes, and the processes of mountain-carving, through the agency of brooks and rivers.
4. Over immense tracts of mountainous regions, rocks, origi-
