238 J. C. MEKRIAM EARTH SCIENCES, BACKGROUND OF J^ISTOl;^ 



is the history of igneous activity evidenced from time to time in the 

 great extrusions of molten material forming successions of lava flows 

 intercalated in the sedimentary series. The history of climate furnished 

 us through a great variety of data gives evidence of almost continuously 

 fluctuating conditions in the physics of the atmosphere ranging between 

 high and low humidity, and between temperatures comparable to those 

 of the glacial periods and the climate of tropical or subtropical regions 

 of the present day. The salient features of climatic history are the con- 

 tinuous change and the evidence of comparatively slight range of tem- 

 perature for the earth as a whole within the span of geologic time as 

 known. 



Earth history, as we see it, shows from the most remote periods to the 

 present constantly varying surface conditions dependent upon an unstable 

 <;rust; continents and mountains arise only to be subject to the steady 

 grind of erosion, wearing them away and spreading the debris over the 

 Kcas. Always do we find land areas and seas, but with much variation 

 as to size and form; always with temperature near that of the present, 

 though fluctuating from warmer and more humid to climates like that .)f 

 the Glacial Period. 



Within the whole span of geological history and its continuous changes 

 recorded, the phases of purely physical history presented do not show us 

 in any of their various aspects definite progression or trend which may 

 be described as an evolutionary process. It was once our practice as 

 geologists to place emphasis on the gegological history of the earth as 

 the continuation of a graded or evolution series based on the succession 

 of stages described in the nebular hypothesis. According to this view, 

 we seemed to see in climatic evolution a gradual movement away from 

 the conditions of the primitive heated earth and toward the present 

 temperature of a cooling sphere. We once thought we saw the early 

 atmosphere fit only for lower organisms and later cleared and purified 

 for the higher types of life. With better understanding of climatic 

 history, it comes out more and more distinctly that while the earth'.:? 

 climate fluctuated continuously, there is no clear evidence of definite 

 progression through a series of stages dependent on gradual cooling of 

 a once highly heated globe. 



So in other phases of purely physical history we have worked out 

 what seemed at first to be evolution series, which have all proved finally 

 to be nothing more than cycles that may be represented by somewliat 

 variable formulae. As nearly as we can determine, the physical history- 

 of the earth within the span of time represented by our legible recoL'd 

 has been so nearly stabilized as to show little or no variation which may 

 not he considered merelv as fluctuation. 



