60 ROCK-METAMORPHISM. 



CHAPTER XL 



ON THE ORIGIN OF THE MINERALS CHARACTERISTIC OF 

 OPHITES AND RELATED ROCKS, THE MINERAL PERI- 

 DOTE IN PARTICULAR. 



THE object of this Chapter is to show that the minerals referred 

 to in the heading are the direct products of hydrothermal 

 reactions in methylosed and volcanic rocks, also of pseudomor- 

 phism in minerals characteristic of ordinary metamorphics and 

 different kinds of igneous masses. If this object can be accom- 

 plished, it will necessarily follow that all the minerals under 

 notice are, in a certain sense, secondary products. Several of 

 these minerals, such as calcite and the different serpentines, are 

 generally admitted to have had this origin; there are others, 

 however, which, as far as we can ascertain, have never been 

 considered any thing else than original in the same sense as the 

 quartz, feldspar, and hornblende of granites are held to be so. 



At first sight the occurrence of secondary minerals in volcanic 

 rocks seems improbable. Still it is pretty generally considered 

 that many of the minerals found in dolerites, trachytes, and 

 ordinary lavas have been generated under circumstances, as to 

 time and conditions, totally different from those under which the 

 rocks in question were formed. 



Even granite, generally assumed to consist of original minerals, 

 occasionally contains zeolites, which all must admit to be 

 secondary products. The same must be asserted of calcite and 

 serpentine, already notified as present in the diorite and other 

 rocks near Galway and in Jersey. 



It is necessary to mention, in the next place, that numerous 

 instances must occur of a rock that has undergone partial me- 

 thylosis, having some of the minerals it contained in its previous 

 or primary condition (whether such rock be igneous or ordi- 

 nary metamorphic) retaining their original character, and others 

 partially altered. Thus the hemithrene of St. Philippe carries 

 hornblende and muscovite ; but as these minerals lie immediately 

 adjacent to the contact rock, viz. gneiss, there can be no doubt 

 that they have escaped the changes which converted the same 

 species accompanying them into malacolite and phlogopite. 



