Presidential Address. 219 



way, and piling this on the surface, must soon become too 

 small for its shell, is apparent ; but when and where would 

 the collapse, crushing, and wrinkling inevitable from this 

 cause begin ? When they began is indicated by the lines 

 of mountain-chains which traverse the Laurentian districts; 

 but the reason why is less apparent. The more or less 

 unequal cooling, hardening and conductive power of tbe 

 outer crust we may readily assume. The driftage un- 

 equally of water-borne detritus to the south-west by the 

 bottom currents of the sea is another cause, and, as we 

 shall soon see, most effective. Still another is the greater 

 cooling and hardening of the crust in the polar regions, 

 and the tendency to collapse of the equatorial protuberance 

 from the slackening of the earth's rotation. Besides these, 

 the internal tides of the earth's substance at the times 

 of solstice would exert an oblique pulling force on the crust, 

 which might tend to crack it along diagonal lines. From 

 whichever of these causes or the combination of the whole, 

 we know that, within the Laurentian time, folded portions 

 of the earth's crust began to rise above the general surface, 

 in broad belts running from N.E. to S.W., and from N.W. to 

 S.B., where the older mountains of Eastern America -and 

 Western Europe now stand, and that the subsidence of the 

 oceanic areas, allowed by this crumpling of the crust, per- 

 mitted other areas on both sides of the Atlantic to form 

 limited table-lands. 1 This was the commencement of a pro- 

 cess repeated again and again in subsequent times,and which 

 began in the middle Laurentian, when for the first time we 

 find beds of quartzite, limestone, and iron ore, and graphitic 

 beds, indicating that there was already land and water, and 

 that the sea, and perhaps the land, swarmed with forms of ani- 

 mal and plant life, unknown, for the most part, now. Inde- 

 pendently of the questions as to the animal nature of Eozoon, 

 I hold that we know, as certainly as we can know anything 



1 Daubree's curious experiments on the contraction of caoutchouc 

 balloons, partially hardened by coating with varnish, show how 

 small inequalities of the crust, from whatever cause arising, might 

 effect the formation of wrinkles, and also that transverse as well 

 as longitudinal wrinkling might occur. 



