PRIMARY AND TRANSITION MOUNTAINS. 



81 



has been stated, in the preceding chapter, that granite by becoming 

 finer grained, frequently passes to the state of porphyry. The eurite 

 of the French geologists, and the weiss-stein or white-stone of Wer- 

 ner, is a granite in which the felspar is the principal constituent part, 

 and is either finely granular or nearly compact. To this variety Eng- 

 lish geologists give the name of compact felspar: the white elvan of 

 the Cornish miners is a porphyritic eurite. 



Geologists have described four formations of porphyry, but it is 

 generally agreed that there is much uncertainty with respect to the 

 situation of these formations. The porphyry which occurs regularly 

 imbedded in granite, or which appears to be formed by a mere change 

 of structure in that rock, may properly be classed with primary rocks : 

 it is not considered to be an extensive formation ; the white elvan of 

 Cornwall, and probably the porphyry associated with mica-slate in 

 Argyleshire, belong to this formation. Porphyry also occurs in enor- 

 mous masses, sometimes intersecting and sometimes covering prima- 

 ry mountains. The granite of Ben Nevis in Scotland is intersected 

 by veins of porphyry ; and at the head of Glen Ptarmagan, a clifF 

 of porphyry 1500 feet high, shaped like an oblique truncated pyra- 

 mid, passes through granite.^ Porphyry, imbedded in transition 

 rocks, or associated with trap or volcanic rocks, must, generally, be 

 regarded as cotemporaneous with the formations in which it occurs. 

 Porphyry, is in some instances, an undoubted volcanic formation, 

 and presents a connecting gradation between granitic primary rocks, 

 and those of a more recent igneous origin. Wherever porphyry 

 occurs unconformably, covering other rocks, it is evidently, more re- 

 cent than the rocks on which it rests, and must be classed with ba- 

 saltic or trap-rocks; this porphyry will be described with them in a 

 subsequent chapter. 



Before taking leave of the rocks classed as Primary, it may be 

 proper to notice that some of the rocks associated with granite, gneiss, 

 and mica-slate, occur also in the transition class, and even in the 

 lower secondary strata. The same causes by which they were form- 

 ed among primary rocks have also operated at a later period : indeed, 

 one of the well known rocks, limestone, has been deposited or form- 

 ed in all the different classes of rocks except the volcanic, and must 

 therefore receive its name from the class with which it is associated ; 

 as primary limestone, transition limestone, &:c. In some instances, 

 the mineral characters, or the fossils, serve to distinguish rocks of the 

 same kind, that occur in the different classes or formations : thus, the 

 rocks associated with primary rocks are generally harder and more 

 crystalline than the same species of rock which occurs in the second- 

 ary class; but this is not invariably the case. 



* Phil. Mag. 

 11 



