8b PKOF. J. W. JT7DD ON TEETIAEY 



arises from tlie fact that the enstatites, as a rule, contain a larger 

 proportion of the magnesian silicates than do the augites. The traces 

 of the secondary foliation of the enstatites are also visible in their 

 alteration-products, in the form of plates and patches of iron-oxides 

 arranged in parallel bands. 



The olivines by simple hydration pass into serpentine ; and this 

 change commences along the surfaces of the grains and penetrates 

 along the lines of cracks which traverse them ; from these lines it 

 extends inwards till not a vestige of the original olivine is left. 

 When, as in the more deeply seated rocks, magnetite has been 

 separated by schillerization, this magnetite remains in the serpen- 

 tine and, by the forms it assumes, often affords a clue to the nature 

 of the original mineral. The hydration of the olivine and its con- 

 sequent conversion into serpentine is attended with an increase in 

 bulk of the individual grains ; in consequence of this, when, as in 

 troctolite (Forellenstein), the grains of olivine are surrounded 

 with felspar-crystals, these latter become cracked, and the fissures 

 thus produced are injected with the serpentinous material. This 

 effect is admirably seen in some cases where each grain of olivine 

 is found to be surrounded by a radiating series of branching fissures, 

 each of which is filled with green serpentinous material (see Plate 

 YII. fig. 7). Even in the ordinary gabbros, where no such 

 regular radiating cracks can be detected, the expansion of the ser- 

 pentine has sufficed to cause its injection into all the accidental 

 fissures and openings in the surrounding minerals. 



The change of titano-ferrite into leucoxene (or titanomorphite) 

 is well known, and is frequently exemplified in these rocks. The 

 chromite and picotite by their decomposition give rise to chromates 

 of a very rich and characteristic green tint. 



The glass of these basic rocks through the action of atmospheric 

 waters is often seen to be converted into a green substance very 

 distinct in appearance from the brown product (palagonite) resulting 

 from the action of heated waters on the same glass. 



As the result of these changes in their constituent minerals, the 

 several members of the basic series are converted into new rocks. 

 The gabbros pass into saussurite-gabbros, wurlizites, or variolites ; 

 the dolerites assume the characters of some of the rocks known as 

 diabases, and the basalts acquire the peculiarities of melaphyres, 

 basaltites, and palatinites. Each of these rocks in turn may, by still 

 further atmospheric change, be reduced to masses of dark-coloured clay 

 or wacke. Of course the slightly altered forms of these rocks are 

 more common in the formations of Tertiary age, and the highly 

 altered forms amoDg older deposits ; but neither the one nor the other 

 can be regarded as characteristic of any particular geological period. 



§ 5. Effects of Crushing Movements in Rock-masses. 



It is only necessary in this place to mention this last and very 

 potent cause of change in the rocks of the basic series. While the 

 action of the former causes is, in the main, destructive, that of the 



