EXCEPTIONAL PHEXOMESA. 327 



nebulous mass from which our system has doubtless been 

 evolved. The retrograde motions of the satellites of 

 Uranus constituted a distinct breach in this law of uniform 

 direction, which became all the more interesting when the 

 single satellite of Neptune was also found to be retro- 

 grade. It now became probable, as Baden Powell well 

 observed, that the anomaly would cease to be singular, 

 and become a case of another law, pointing to some 

 general interference, which has taken place on the bounds 

 of the planetary system. Not only have the satellites 

 suffered from this perturbance, but Uranus is also 

 anomalous in having an axis of rotation lying nearly in 

 the ecliptic ; and Neptune constitutes a distinct exception 

 to the empirical law of Bode concerning the distances of 

 the planets, which exceptional circumstance may pos- 

 sibly be due to the same disturbance. 



Geology is a science in which accidental exceptions are 

 very likely to occur. Only when we find strata in their 

 original relative positions, can we surely infer that the 

 order of succession is the order of time. But it not 

 uncommonly happens that strata are inverted by the 

 bending and doubling action of extreme pressure. Land- 

 slips may carry one body of rock into proximity with an 

 unrelated series, and produce results apparently inex- 

 plicable 1 . Floods, streams, icebergs, and other casual 

 agents, may occasionally lodge remains in places where 

 they would be wholly unexpected. 



Though such interfering causes may have been often 

 wrongly supposed to explain important discoveries, the 

 geologist must of course always bear the possibility of 

 interference in mind. Scarcely more than a century ago 

 it was yet held by many persons that fossils were acci- 

 dental productions of nature, mere forms into which 

 minerals had been shaped by no peculiar cause. Voltaire 



1 Murchisoii's ' Silurian System,' vol. ii. p. 733, &c. 



