314 EVOLUTIONAL GEOLOGY. 



somewhat laige island. What area of this island, we may ask, would 

 suffice to supply the Cambrian sediments of Wales and Shropshire? 

 Admitting that the area of denudation was ten times as large as the area 

 of deposition, its dimensions are indicated by the figure a h c d on the 

 chart. This evidently leaves room enough on the island to furnish all 

 the other deposits which are distributed along the western shores of 

 the Cambrian Sea, while those on the east are amply provided for by 

 that portion of the European Continent which then stood above water. 



If 1 foot in a century be a cjuantity so small as to disappoint the 

 imagination of its accustomed exercise, let us turn to the Cambrian 

 succession of Scandinavia, where all the zones recognized in the British 

 series are represented b}' a column of sediment 21^>0 feet in thickness. 

 If 1,600,000 years be a correct estimate of the duration of Cambrian 

 time, then each foot of the Scandinavian strata must have occupied 

 5,513 3'ears in its formation. Are these figures sufficiently incon- 

 ceivable ? 



In the succeeding system (that of the Ordovician) the maximum thick- 

 ness is 17,000 feet. Its deposits are distributed over a wider area than 

 the Cambrian, but they also occupied longer time in their formation; 

 hence the area from which they were derived need not necessarily have 

 been larger than that of the preceding period. 



Great changes in the geography of our area ushered in the Silurian 

 system. Its maximum thickness is found over the lake district and 

 amounts to 15,000 feet, but in the little island of (xothland, where all 

 the subdivisions of the system frojii the Landover}' to the Upper Lud- 

 low occur in complete sequence, the thickness is only 208 feet. In 

 Gothland, therefore, accoixling to our computation, the rate of accu- 

 mulation was 1 foot in 7,211 years. 



With this example we must conclude, merely adding that the same 

 story is told by other systems and other countries, and that so far as 

 m}^ investigations have extended I can find no evidence which would 

 suggest an extension of the estimate I have proposed. It is but an 

 estimate, and those who have made acquaintance with "estimates''' in 

 the practical affairs of life know how far this kind of computation may 

 guide us to or from the truth. 



This address is already unduly long, and yet not long enough for the 

 magnitude of the subject of which it treats. As we glance backward 

 over the past we see catastrophism yield to uniformitarianism, and this 

 to evolution, but each as it disappears leaves behind some precious 

 residue of truth. For the future of our science our ambition is that 

 which inspired the closing words of your last president's address, that 

 it may become more experimental and exact. Our present watchword 

 is Evolution. May our next be Measurement and Experiment, Experi- 

 ment and Measurement. 



