PRIMARY AND SECONDARY GRANITE. 



65 



when the felspar and quartz predominate, it becomes again massive 

 or common granite. 



What has been said respecting the alternation of gneiss and gran- 

 ite, will apply to the alternation of granite and mica-slate. In the 

 latter, the felspar is wanting ; but if it re-appear, it becomes either 

 granite or gneiss. Mica-slate also passes, by such insensible grada- 

 tions, into slate, that the occasional occurrence of granite in some 

 ancient slate-rocks, may admit of a similar explanation. We shall 

 thus sweep away the secondary granites, which have so much be- 

 wildered the systems of many geologists : indeed nothing can ap- 

 pear more puerile and trifling than the labour of making distinctions, 

 where nature has made none. Of this we have an instance in the 

 distinctive characters which have been given of primary and second- 

 ary granite. 



Primary Granite. 



1. Sometimes red. 



2. Contains garnets. 



3. Isj sometimes, porphyridc. 



Secondary Granite. 



1. Felspar, commonly a deep red. 



2. Contains garnets. 



3. Not porphyritic; but, according 



to Professor Jameson, is some- 

 times porphyritic. 



Again, M. D'Aubuisson tells us, that the colour of primary granite is 

 almost always white. 



What has been advanced may be sufficient to prove, that the at- 

 tempts to distinguish primary from secondary granite by their miner- 

 al characters, are worse than useless ; as they waste the time of the 

 learner, and tend to disgust him with a science already too heavily 

 burdened with unmeaning terms and frivolous distincdons. 



There is a particular form of granite, in which the constituent parts 

 are so minute and so intimately mixed, that it appears very minutely 

 granular or even compact : to this variety the French geologists have 

 given the name of Eurite ; it has generally been described by Eng- 

 lish geologists as Compact felspar, into which it passes by insensible 

 gradations. This rock frequently, contains imbedded crystals of fel- 

 spar, and forms what has been denominated felspar-porphyry. In 

 Cornwall, it occurs in beds in common granite ; but, instead of be- 

 ing regarded as a different rock, it may be more properly classed 

 by the geologist with granite, being only a variety, in which felspar 

 greatly predominates. This rock occurs, also, in an unconformable 

 position, and is, generally, described as porphyry, and appears to 

 form a connecting link between common granite and the compact 

 varieties of volcanic porphyry, with a base of felspar called by the 

 French Trachyte. 



Sienitic granite, in which the mica is, partly or entirely, replaced 

 by hornblende, occurs in some situations in the same bed with com- 

 mon granite, and, therefore, must be regarded as a variety of gran- 

 ite. I have frequently observed instances of this change from gran- 



