862 



THE WONDERS OF GEOLOGY. Lect. VIII. 



result of incipient decomposition, for the fissures become 

 enlarged by exposure to the air and water, and the rock 

 separates into masses resembling piles of masonry, of which 

 the celebrated Logan or Rocking-stones, and the Cheese- 

 wring of Cornwall, are examples.* 



In some instances, a tendency to a columnar arrange- 

 ment is observable, as in the cliffs near the Land's End, 

 in Cornwall. The granitic porphyry of Corsica (Napo- 

 leonite) presents an orbicular structure, in which balls 

 or spheroids of concentric and alternate coats of horn- 

 blende and compact felspar, are disseminated with much 

 regularity throughout the mass. 



The granites of Devonshire and Cornwall are considered 

 by Sir H. de la Beche to have been protruded after the depo- 

 sition of the coal-measures of Devon, and antecedently to 

 the Triassic series. " They appear to have been thrown up 

 through points of least resistance, in a line extending from 

 the southern part of Devonshire to the Scilly Isles, part 

 having protruded through the weakest places, and the 

 remainder being concealed beneath. From the Scilly Isles 

 to Dartmoor inclusive, there seems to have been the 

 upthrust of one mass, which found points of less resistance 

 amid the superincumbent accumulations, more in some 

 places than in others. As the masses rose, the edges of the 

 detrital, trappean, and calcareous beds against which they 

 pressed, were frequently fractured, and into these fractures 

 the granitic matter was forced, forming veins which 

 can often be traced terminating in fine threads ; so that 

 not only was the pressure great, but the fluidity of 

 the igneous rock sufficient to pass into small rents and 

 crevices." f 



A group of plutonic rocks, consisting of granite and 

 syenite protruded through overlying schistose and carboni- 



* See Appendix B, on the Logan-stones. 



f Memoirs of the Geological Survey of Great Britain. 



