PHILOSOPHICAL SOCIETY OF WASHINGTON. 47 



trade-winds, must also affect considerably the position of the 

 median zone, or the thermal equator. As .to the effect of land 

 distribution on climate, this latter is perhaps even more controlled 

 by continental configurations (which determine the deflections of 

 ocean currents) than by merely areal distributions. 



In reply to Mr. Powell, in whose remarks on the chronology 

 of post-tertiary erosion Mr. T. felt great interest, he would only 

 say that he did not think that Mr. Croll's theory imposed any 

 such narrow limits of time on the geologist as had been sup- 

 posed. The variations of orbital eccentricity of our planet, de- 

 pending on the conjoined action of the larger and nearer planets, 

 with incommensurable periods, are, of course, very irregular, both 



in time and amount; and although they may be said to occur 



roughly at periods of one or two hundred thousand years— yet 

 the extreme or exceptional ranges of eccentricity (which alone 

 may be supposed sufficient to produce any very decided results) 

 are found to occur only at periods of one or two million years 

 apart. Thus, if we suppose, with Mr. Croll, that our last glacial 

 period, or rather double period, coincided in time with the eccen- 

 tricities culminating two hundred and ten and one hundred 

 thousand years ago, the very notable maximum of eccentricity 

 occurring eight hundred and fifty thousand years ago may repre- 

 sent, as suggested by Mr. Croll, a glacial period suspected to 

 have existed in the upper Miocene. The next preceding notable 

 maximum occurred about two and a half million years ago. 



It was erroneous to suppose, as had been intimated, that con- 

 glomerates alone had been accepted as evidencing past glacial 

 action. Professor Ramsay, twenty years ago, urged the glacial 

 origin of the Permian breccias, from the large size of the frag- 

 ments, from the rarity of rounded pebbles, from the angular and 

 flat faced form of most of the constituents, and, lastly, from the 

 polished and grooved surfaces not unfrequeutly found among 

 them. 



Mr. Gill said, that, so far as he, a biologist, had to pass on the 

 question at issue, the 210,000 years plus the antecedent period 

 of the Pleiocene was sufficient ; it must be remembered, however, 

 that the animal life of the earth had remained practically un- 

 changed (except as to geographical distribution of forms) from a 

 period long antecedent to the glacial epoch. But the fauna of 



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