298 NATURAL SCIENCE. April, 1894. 



predominate. Gypsum is a characteristic mineral. There is strong 

 evidence of inland salt lakes, lagoons, and tidal shores. 



It is often said, and with great force, that it is impossible to 

 determine the age of a rock or the formation to which it belongs by 

 its lithology. That is true, looking at the rocks separately and indi- 

 vidually, for the same individual conditions occur in all the ages, but 

 notwithstanding this there are broad lithological resemblances or 

 groupings, what is called fades when applied to the organic remains. 

 There are coal beds in the Western States of North America, but no 

 one would mistake the Laramie or Tertiary rocks of those countries 

 for Carboniferous any more than the Oolite rocks in Scotland, in which 

 coal is found, could be mistaken for the Coal-measures ; nor yet the 

 Lias, though ironstone is found and worked to a large extent in the 

 Cleveland district of Yorkshire. There may be land and sea con- 

 ditions and deep-sea conditions represented in the rocks of any age, 

 but independently of the fossils there are characteristic rock-groups 

 which distinguish the one period from the other. 



Even so superficial a stratum as the Drift has common charac- 

 teristics over Britain, Europe, Asia, and North America, a fact which 

 does not merely point towards analogous climatic conditions but 

 towards similar physiographic conditions on a large scale. Indeed some 

 geologists think the Glacial period resulted from the physiographic 

 conditions which then obtained. Then again, to come towards 

 historic time, the post-Glacial deposits that fringe our coasts, the 

 estuarine clays and forest-beds have a striking resemblance over very 

 large areas, and even continents. Indeed, so important seemed these 

 characteristic similarities and differences to our predecessors in the 

 science of Geology, that they constructed, wiped out, and reconstructed 

 the earth's surface and crust again and again by the aid of cataclysmic 

 convulsions. The influence of Lyell was all towards showing 

 the fallacy of these inferences, but then the pendulum swung too 

 much the other way, and we were led to dwell inordinately on 

 continuity and recurrence. 



The uniformitarian theory was a great boon in bringing men's 

 minds down from the region of pure imagination to that of fact. 

 It was a good working hypothesis, but, like most generalisations, was 

 a little one-sided. It represented too exclusively one side of the 

 phenomena of the earth's history to the neglect of another — that of 

 development. I am not one of those who think the doctrine of 

 uniformity a " fetish," but rather a theory that kept us within the 

 range of practical fact. For my part, I am prepared to accept any- 

 thing that can be proved, but until the proof is forthcoming we are on 

 safe ground if we confine ourselves to agencies of which we have 



experience. 



T. Mellard Reade. 



(To be continued.) 



