CRITICAL PERIODS IN THE HISTORY OF THE EARTH. 471) 



matter entirely new? Evidently the suddenness of the change in organic 

 forms is only apparent. If we could recover the record, which was doubt- 

 less carried on elsewhere, the break would disappear ; if Ave could find the- 

 missing leaves the reading would be continuous. In every such instance, 

 therefore, there is a lost interval of history. In cases of local unconformity 

 we recover the lost record in other places, and thus fill up the blank in the 

 history. But in some cases of very general unconformity, such as those 

 which mark the great divisions of time, the loss is not yet recovered, per- 

 haps is irrecoverable, though doubtless the more complete knowledge of the 

 geology of the whole earth surface will go far toward filling blanks and 

 making the record continuous. 



The view above presented is now held by all geologists, but there seems 

 to be danger under the influence of the now dominant views of evolution, 

 of erring on the other extreme. Assuming a uniform rate of evolution, 

 many, it seems to me, commit the mistake of measuring the amount of lost 

 interval by the amount of change of organic forms, and thus discredit the 

 real value of the geological record by exaggerating greatly its fragnientarj'" 

 character. On the contrary, there appears good reason to believe that the 

 evolution of the organic kingdom, like the evolution of society and even of 

 the individual, has its periods of rapid movements and its intervals of compara- 

 tive repose and re-adjustment of equilibrium. Cxeological history, like all 

 .)ther history, has its periods of comparative quiet, during which the forces 

 of change are gathering strength, and periods of revolution, during which 

 the accumulated forces manifest themselves in conspicuous changes in phys.. 

 ical geography andclimate, and therefore in rapid movement in the march of 

 •evolution of organic forms — periods when the forces of change are potential, 

 and periods when they become active. Conformable rocks represent the in- 

 tervals of comparative quiet, during which organic forms are either j^erma- 

 nent or change slowly ; unconformity represents a time of oscillation, with 

 increase and decrease of land, and therefore of rapid changes of physical 

 conditions and correspondingly rajiid movement in evolution. The general 

 xmconformities, of course, mark times of very general commotion — of wide- 

 spread changes of physical geography and climate, and consequently of 

 exceptionally rapid and profound changes in organic forms. 



These periods of revolution in all history are critical, and hence are of 

 especial interest to the philosophic historian and to the evolutionist ; but 

 they are also in all history periods of lost record. And as in human so also 

 in geological history — the farther back we go the longer are the lost inter- 

 vals and the more irrecoverable the lost records. "We will now give exam- 

 ples of such lost intervals, and show their significance in evolution. 



The first and by far the greatest of these is that which occurs between 

 the Archaean and the Palaeozoic. In every part of the earth where the con- 

 tact has yet been observed the Primordial lies tinconformably on the up- 

 turned and eroded edges of the Archaean strata. This relation was observed 

 first in Canada, then in various jDarts of the United States, then in Scotland. 



