796 



SCIENCE. 



[N. S. Vol. XII. No. 308. 



thick ; still a little less rapidlj' towards the 

 Malverns, where they are onlj^ 800 feet 

 thick ; and most slowly towards St. David's 

 Head, where they are 7,400 feet thick. The 

 Cambrian rocks of Wales were in all prob- 

 ability the deposits of a river system which 

 drained some vanished land once situated 

 to the west. How great was the extent of 

 this land none can saj^; some geologists 

 imagine it to have obliterated the whole or 

 greater part of the North Atlantic Ocean. 

 For my part, I am content with a somewhat 

 large island. What area of this island, we 

 may ask, would suffice to supplj' the Cam- 

 brian 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 b c don the chart. This evidentlj' 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 pro- 

 vided for by that portion of the European 

 continent which then stood above water. 



If one foot in a centurj'' be a quantity 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 by a column of sedi- 

 ment 290 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 years in its formation. Are these 

 figures sufficiently inconceivable? 



In the succeeding system, that of the 

 Ordoviciau, the maximum thickness is 17,- 

 000 feet. Its deposits are distributed over 

 a wider area than the Cambrian, but they 

 also occupied longer time in their forma- 

 tion ; 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 Gothland, where all 

 the subdivisions of the system, from the 

 Landovery to the Upper Ludlow, occur in 

 complete sequence, the thickness is only 

 208 feet. In Gothland, therefore, according 

 to our computation, the rate of accumula- 

 tion was one foot in 7,211 years. 



With this example we must conclude, 

 merely adding that the same story is told 

 bj' other sj'stems and other countries, and 

 that, so far as my investigations have ex- 

 tended, 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 ' esti- 

 mates ' in the practical affairs of life will 

 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 backwards over the past we see 

 catastrophism yield to uniformitarianism, 

 and this to evolution, but each as it disap- 

 pears 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 Evolu- 

 tion. May our next be Measurement and 

 Experiment, Experiment and Measurement. 



W. J. SOLLAS. 



THE INTERNATIONAL CONGMESSES OF ME- 

 TEOROLOGY AND AERONAUTICS 

 AT PARIS. 

 These Congresses were held nearly simul- 

 taneously on account of their allied inter- 

 ests. The Meteorological Cougi-ess, which 

 began its sessions on September 10th, had the 

 same character as the Congress held during 

 the Paris Exposition of 1889, that is to say, 



