The Paragenetic Relations of Minerals. 89 



mention the occurrence of broken felsite crystals in granite, 

 and Noggerath has observed them in trachyte. In a lava at 

 Etna, crystals of pyroxene have been found collected together 

 at the under surface, as if they had sunk. However, it is 

 probable that these facts will not admit of any other infer- 

 ence than that there was some interval between the soldifica- 

 tion of the matrix and that of the imbedded substances. On 

 the other hand, there are mixtures of minerals presenting a 

 porphyritic appearance, of which the matrix is undoubtedly of 

 much later date than the minerals it contains. Quartz very 

 frequently contains tourmaline, sometimes in crystals, though 

 never perfect ones. The terminal planes at one end of the 

 crystal may be perfect, but the other end is always broken. 

 It is most frequently found in fragments, sometimes even 

 curved and cracked, and the quartz is found to adhere more 

 strongly to the electro-negative pole of the fragments. When 

 there is a preponderance of tourmaline in this mixture, it is 

 called tourmaline rock or schorl. It often contains cassite- 

 rite very finely disseminated throughout its mass ; and, be- 

 sids other localities, it occurs in Cornwall. In this mixture, 

 the quartz must be the more recently formed, for whenever 

 definite crystals of quartz and tourmaline are associated in 

 druses, the quartz is always implanted upon the tourmaline. 

 It is therefore extremely probable that this rock was formed 

 by a deposition of silica in an opaline condition around 

 tourmaline crystals, and that on its subsequent conversion 

 into quartz, the crystals were broken by the contraction of 

 the siliceous mass. 



Fragments of some species of epidote occur imbedded in 

 quartz in a precisely similar manner. The contortions and 

 fractures of these fragments are sometimes very marked, as 

 in the magnesian epidote of St Marcel, the zoisite of the 

 Tyrol and Carinthia, &c. It is further probable that the 

 so-called epidote rock is precisely analogous in point of for- 

 mation to the above-mentioned tourmaline rock. Quartz, 

 when associated with epidote in crystals, is always implant- 

 ed upon it. The epidote and quartz veins in the diorite of 

 I Neustadt (Saxony), of the Labyrinth Berge (Bavaria), the 

 druses from Arendal (Norway), Bourg d'Oisans, Pitkarand 



