DYNAMICAL GEOLOGY. 197 



France was even more reserved towards this aspect of Lyell's 

 work. The ideas of Cuvier were deeply rooted, and were ably 

 supported by Elie de Beaumont and Alcide d'Orbigny. It 

 was not until after the death of these two gifted scientists that 

 the Uniformitarians could become successful. Many of Lyell's 

 opinions, more especially his theories regarding crystalline 

 schists, were warmly contested, and his explanation of vol- 

 canic phenomena and mountain-making was afterwards found 

 insufficient. At the same time, the leading principle of his 

 geological teaching that the key to the solution of the events 

 of the past is to be found in the study of the natural 

 forces still acting has remained as the secure basis of 

 all modern geological investigation. The recognition of 

 this grand principle gave a new significance to dynamical 

 geology, and brought it at once into prominence among 

 geologists. 



Sir Henry de la Beche wrote in 1835 an excellent in- 

 troduction to dynamical geology, entitled Hoiv to Observe; 

 in later editions, the title was changed to The Geological 

 Observer. De la Beche followed essentially the same 

 method as Lyell, and his book, which is full of new obser- 

 vations and facts, may almost be regarded as a supplement 

 to Lyell's Principles. 



A. Geological Action of the Atmosphere. The destructive and 

 constructive activity of the atmosphere plays in general but a 

 small part in the conformation of the earth's surface, and was 

 for a long time neglected by geologists. Chemical effects can 

 only be produced by the atmosphere in its combination with 

 water or living organisms. Mechanical forms of destruction 

 are effected by the atmosphere in all regions subject to marked 

 extremes of seasonal or diurnal temperature, the wasting of 

 the rocks being considerably aided by the strain of alternating 

 expansions and contractions. The geographer Livingstone 

 was the first who observed that in the African deserts sharp 

 fragments sprang away with a ringing tone from the basalt rock 

 whenever a hot day was succeeded by a night with very low 

 temperature. Other travellers have since confirmed this 

 observation, and have ascertained that the so-called " Ham- 

 mada " region undoubtedly owes its surface-mantle of angular 

 fragments of stone to the destructive effects of rapid variations 

 of temperature. 



