ORIGIN OF MINERALS CHARACTERISTIC OF OPHITES ETC. 67 



been adduced to the contrary are forthcoming, we shall continue 

 to maintain that peridbte, whatever kind of rock may be its 

 matrix, or whether occurring in crystalline forms peculiar to 

 itself or as amorphous masses, is as much a secondary product 

 as other secondary minerals which may be its accompaniments. 



Weighing the evidences and considerations hitherto adduced, 

 it may be taken as clear that, although genetically a secondary 

 mineral, peridote is of independent origin in the crystalline and 

 crystalloidal examples last under notice. 



Our next object is to show that peridote, or a mineral sub- 

 stance assumed to be the same, has originated through pseudo- 

 morphism after other minerals, and that therefore in cases of 

 this kind it cannot be of independent origin. 



That peridote occurs frequently changed into other minerals, 

 as serpentine, is a well-established fact; but the proposition 

 just laid down does not seem to have been entertained by 

 mineralogists : it is one, however, which cannot be said to be 

 gratuitous ; for other minerals are known to be similarly poly- 

 genetic. As hexagonal crystals, quartz is in a form proper to 

 itself and of independent origin ; but as the same mineral sub- 

 stance is found in forms peculiar to calcite, such forms are 

 pseudomorphs, and cannot have originated independently. Our 

 illustration has its parallel in peridote. 



Again, as we purpose to show that peridote is pseudomor- 

 phous after hornblende and augite, we may be allowed before- 

 hand to ask the question As serpentine frequently occurs 

 pseudomorphosed after hornblende and augite, why cannot the 

 latter species be pseudomorphosed into peridote, which is so 

 closely related to . serpentine ? We shall endeavour to answer 

 the question in the affirmative by the consideration of evi- 

 dences pertaining to the crystalline form, cleavage, and 

 polarization respectively characteristic of the three minerals 

 concerned. 



Peridote belongs to the trimetric crystalline system ; and it 

 necessarily has for its primary a right rectangular prism, which 

 in this particular mineral rarely occurs otherwise than under 

 somewhat complex prismatic and basal modifications. Its 

 simplest prismatic modifications, by removal of the lateral 

 edges to the complete effacement of all the primary faces, 

 would be rhombic, with acute and obtuse angles respectively 



