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which each system exhibits in its typical locality. The systems thus serve as known and definite standards of comparison in the construction of the time-scale, as the dominance of nations or the dominance of the dynasties in each serve as time standards for the discussion of ancient human history. As the period of each dynasty in ancient history is marked by continuity in the successive steps of progress of the country, of the acts of the people and of the forms of government, and the change of dynasties is marked by a breaking of this continuity, by revolutions and readjustments of affairs, so in geological history the grand systems represent periods of continuity of deposition for the regions in which they were formed, separated from one another by grand revolutions interrupting the regularity of deposition, disturbing by folding, faulting and sometimes metamorphosing the older strata upon which the following strata rest unconformably and form the beginnings of a new system.

Geological revolutions were not universal for the whole earth; from which it results that these typical systems and their classification are not equally applicable to the formations of all lands. It is important also to note that the geological revolution was not a sudden catastrophe but the culmination of slowly progressing disturbances bringing the surface of the region concerned ultimately above the level of the ocean, the ocean level being a pivotal point in geological rock formation. The area whose surface is below the sea level may be accumulating deposits and making rocks, but so soon as the same region is lifted above the surface it becomes a region of erosion, destruction and degradation. Whenever, therefore, in the oscillations of level any particular part of a continental mass of the earth's crust passes permanently, or for a long geological period of time, above the sea level, a great event in geological history has culminated. In case the elevation is only temporary the event is marked by unconformity, or a break in the continuity of the formations; when it is permanent the geological record for that region ceases except so far as fresh water deposits in lakes may continue independent records. Hence it is that these periods of revolution are of such import-