80 



PORPHYRY. 



ly associated with granite : the passage of one rock into the other 

 by the increase or decrease of felspar, may frequently be observed 

 in the same mountain. When hornblende and felspar are more in- 

 timately blended, they form the rock called by the Germans Green- 

 stone^ by the French Diabase ; and with other rocks of similar com- 

 position, are h^equently described as trap-rocks, and by the French 

 as roches amphiboliques : these will be more properly noticed in the 

 subsequent chapters. When the hornblende and felspar are so close- 

 ly and minutely intermixed that the rock appears homogeneous, the 

 trap has all the external character of a rock (hereafter to be more 

 fully described) called Basalt.* In examining the geological speci- 

 mens of Saussure in the museum at Geneva, I observed, that the 

 rocks which he so frequently mentions under the name of Corneenne, 

 are mixtures of hornblende and felspar, in which the former min- 

 eral predominates. 



Hornblende intermixed with felspar, forming sienite and green- 

 stone, occurs at the Malvern Hills, in Worcestershire ; at the Charn- 

 wood Forest hills in Leicestershire ; and in Cornwall, Cumberland, 

 and North and South Wales. Very little well characterized horn- 

 blende-slate is found in any part of England, but, it occurs abund- 

 antly in the alpine parts of Scotland, and in most of the principal 

 mountain ranges in Europe. The various intermixtures of horn- 

 blende and felspar, to which the name of trap-rocks is frequently 

 given, may more properly be classed with transition rocks. f 



Porphyry derives its name from a Greek word denoting purple; 

 the rock to which it was at first applied had a purple colour. In the 

 modern acceptation of the term, any rock which is compact or fine- 

 ly granular, and contains distinct imbedded crystals, is called Porphy- 

 ry, whatever be its colour. The base or paste of most porphyritic rocks 

 is felspar ; and the imbedded crystals are also felspar, though there 

 may be also small grains or crystals of quartz or other minerals. It 



* The rock to which the French give the name of Diabase, the compact trap 

 of Werner, resembles basalt (which the French call Dolerite) so closely,' both 

 in composition and physical characters, that the division into two species seems to 

 have been made principally to serve the purpose of theory. Diabase is composed 

 of felspar and hornblende, and dolerite of felspar and augite intimately com- 

 bined. But, as hornblende and augite do not differ more in chemical composi- 

 tion, than one species of hornblende differs from another, and as these two min- 

 erals are to be distinguished only by their crystallization ; when they occur un- 

 crystallized, may they not be regarded as identical It is true, augite occurs 

 abundantly in rocks of undoubted igneous origin, and in the lavas of recent vol- 

 canoes ; hornblende occurs also in basaltic lavas, but more frequently, in rocks 

 of which the igneous origin is not so generally admitted : yet it may be fairly 

 doubted, whether the distinction between compact diabase, and compact dole- 

 rite, has not been made in order to form gratuitous conclusions respecting the dif- 

 ferent origin of rocks, which are, in chemical composition and external charac- 

 ters, essentially the same. 



t Dr. Macculloch states an instance in Shetland, where slate (clay-slate) ap- 

 pears to be converted into hornblende-slate by approximating to granite; but no 

 inference can be fairly drawn from a solitary instance of this kind, as there is no^ 

 evidence to prove that the hornblende-schist is not an original rock. 



