284 THE JOURNAL OF GEOLOGY. 



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 dynas- 

 ties is marked by a breaking of this continuity, by revolutions 

 and readjustment 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 revohitiom interrupting the regularity of deposition, 

 disturbing by folding, faulting and sometimes metamorphosing 

 the older strata upon which the following strata rest unconform- 

 ably 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 classi- 

 fication 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 progres- 

 sing 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 sur- 

 face 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 degrada- 

 tion. Whenever, therefore, in the oscillations of level any partic- 

 ular part of a continental mass of the earth's crust passes perma- 

 nently, 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 perma- 

 nent 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- 



