British Mineralogy. 335 



racteristic of certain eruptive rocks but not of others. And 

 amongst the numerous cases ill which other minerals were found 

 to diner when occurring under different circumstances may be 

 mentioned the following. 



Mica, when it occurs in granitic rocks*, was found, as a rule, 

 to present itself as Muscovite, although in some special granite 

 it also does occur as lepidolite. When in serpentines or meta- 

 morphic limestones it was, in all cases examined, phlogopite. 

 When in zircon- syenites or miascite, it is present as astrophyllite 

 or titaniferous mica ; whilst in the volcanic rocks of Vesuvius, 

 Laach, Baikal, &c. it occurs as Biotite. 



• Augite is found to present great differences in chemical com- 

 position and physical characters according as it is respectively 

 found in serpentine, diabase, porphyrites, dolerites, metamorphic 

 limestones, volcanic rocks and aerolites, — and may be in all these 

 cases regarded as so many subspecies or varieties, admitting of 

 easy and decided discrimination. 



Garnet, when met with in granitic rocks, presents itself as 

 almancline, iron garnet, or common garnet. When in connexion 

 with dioritic eruptions and metallic veins it was found to be in- 

 variably as melanite, grossular, or colophonite. When in trap 

 dykes it always presents itself as pyrope; and, as Durocher has 

 observed, when in serpentine, it invariably occurs as magnesian 

 garnet. 



Leonhard, in his Charakteristik dor Felsarten, has also shown 

 that garnets, when in granite, always crystallize in trapezoedra, 

 whilst when in the metamorphic schists it is invariably in the 

 form of the rhombic dodecahedron. In like manner it was found 

 that all the specimens of cuprite encountered in metallic lodes of 

 the postoolitic period, were invariably crystallized as cubes, with- 

 out any trace of bevelment or replacement of the angles, whilst 

 those from lodes of the palaeozoic period appeared invariably as 

 octahedra in all the specimens examined. 



Again, apatite, when occurring in granites, was invariably 

 found to be fiuor-apatite, but, when with the later diorites, was 

 chlorapatite. 



Hornblende differs in chemical composition and crystalliza- 

 tion when respectively found in situ in diorites, zircon-syenites, 

 porphyrites, or metamorphic schists. 



Scapolite, a common mineral in granites, presents itself as 

 meionite in volcanic rocks. 



* By granitic is here understood the old acid rocks containing free 

 quartz. Although not unfrequently used to denote structure, the author is 

 not disposed to admit that there is any actual structure specially characte- 

 ristic of granite when compared to many other coarsely crystallized erup- 

 tive rocks. 



