Introduction 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 surtace of Spain.” 
Guy! satin (fig. 14.), on the authority of Messrs. Coney- 
beare and Phillips, represents part of the primary district of 
Cornwall, 
Pyrogeneous Formations. 
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 outlyi ing masses, sometimes as dy lee, 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 forinations, even through the chalk. Columnar basalt has 
jong excited the wonder even ai those for whom natural science 
has 1 fewcharms. ‘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, par ticularly the limestones, 
VoL. Tl. — No. Ll. F 
