242 



THE GEOLOGIST. 



centre of the disturbing forces, did not, in his^opinion, con« 

 firm the view the authors had taken of the origin of those 

 contortions. Again, Mr. Sedgwick stated, the position of 

 the successive strata in the British chains, was not gene- 

 rally such as that which characterized the chain so carefully 

 described by the authors of the paper. The effects of disturb- 

 ing forces, such as the intrusion of igneous rocks, was chiefly 

 dependent on the nature of the rocks affected. In Cumber- 

 land the porphyritic rocks, which were evidently molten 

 when introduced, had become hard by cooling, and had 

 been fractured and dislocated along with the rocks among 

 which they were intruded; but from the very nature of 

 those rocks, they could not be thrown into many undula- 

 tions. In North Wales, where the conditions differed, and 

 the igneous rocks were less abundant, the alternating beds 

 of solid porphyry and softer rocks were thrown into a series 

 of anticlinal and synclinal lines ; whilst in the Liege country 

 the beds, when in a very soft and plastic state, had evi- 

 dently been subjected to great lateral pressure, forcing them 

 to assume enormous contortions, but never elevating them 

 into mountains. The authors had, he thought, rather un- 

 dervalued the power of tangential forces. These were well 

 illustrated in the effects produced upon the soft slates of 

 North Devon, by the intrusion of masses of granite many 

 miles across, like that forming the forest of Dartmoor, 

 between which and other granite masses, the strata were 

 crumpled and thrown into innumerable undulations. He 

 believed there was very little analogy between the pheno- 

 mena produced by earthquakes, and those attributed to con- 

 tinental elevation; the oscillations of the earth's surface 

 produced by earthquakes were like those of a cord struck 

 when subjected to tension : from the very nature of these 

 vibrations, they might be propagated rapidly over a great 

 part of the globe. The impulses of elevation, as far as any 



