Introductioii to Geology. 



65 



in, in calm weather, from the summit of the Piton, a surface 

 of the globe of 5700 square leagues, equal to one fourth of 

 the surface of Spain." 



Our section {Jig. 14.), on the authority of Messrs. Coney- 

 beare and Phillips, represents part of the primary district of 

 Cornwall. 



PyfVgeneous Forinations, 



Basaltic Rocks, Trap, and Porphyritic Rocks, of supposed 

 igneous origin, and of various degrees of antiquity, are asso- 

 ciated alike with the transition and primary, and often mate- 

 rially influence the position of the most recent deposits. They 

 are unconformable and unstratified, appearing sometimes in the 

 form of outlying masses, sometimes as dykes, traversing and 

 intersecting numerous rocks, and occasionally forming beds 

 or masses, which alternate with other formations. They often 

 cap the summits of the primitive mountains ; and, on the Andes, 

 vast masses of porphyry and basalt are described by Hum- 

 boldt as " arranged in the form of regular and immense 

 columns, which strike the eye of the traveller, like the ruins 

 of enormous castles lifted into the air." 



No organic remains have been decidedly found in these 

 rocks. Basaltic dikes impede the progress of the miner, dis- 

 turb the position of the coal measures, alter the nature and 

 structure of the adjacent rocks, and penetrate sometimes into 

 higher formations, even through the chalk. Columnar basalt has 

 long excited the wonder even of those for whom natural science 

 has few charms. The magnificent specimens in the north of Ire- 

 land and in the Western Islands of Scotland are of this class. 



The beds of toadstone and whinstone of Derbyshire, and 

 the whinsill of Northumberland, are evidently rocks of the 

 basaltic series. 



Under this head the accumulations of volcanic matter, of 

 all ages, seem to be properly referable. 



Transition. 



Transition, or Intermediate, is that order of rocks which 

 approximates, both in geological position and in mineralogical 

 character, to the primary. They all bear traces of organic 

 remains ; some sparingly, others, particularly the limestones. 



Vol. III. — No. 11. f 



