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CHAP VI.

HISTORICAL VIEW OF THE STRATIFIED ROCKS IN THE CRUST OF THE EARTH.

IN describing the successive phenomena visible in the crust of the earth, we may either begin at the surface, and pass from the operations of to-day, through the monuments of changes performed in historic periods, to those of earlier date; and thus, proceeding from the known to the unknown, approach by an easy gradation to the remote eras and obscure conditions of our planet, which were once degraded by the misapplied title of "Chaos;"—or take our departure at the most ancient recognisable point of geological time, and trace the events which happened on the globe in the order of their occurrence.

The former process offers some advantages to the student who, unaided by or distrustful of the generalisations already arrived at, is desirous of acquiring by his own labours a correct view of the relation of the present to earlier conditions of the globe: for thus, proceeding from diurnal operations to primeval phenomena, he is able to classify his observations with reference to causes really acting, to assemble partial truths into laws of phenomena, and by mere comparison of these with the actual condition of nature to arrive at the knowledge he is in quest of.

But for the purpose of clearly unfolding the series of geological phenomena whose laws are known, the contrary method is to be preferred. To describe what is known of the structure of the earth in the order of the occurrence of the phenomena—to present a series of pictures of its successive conditions—to exhibit these