64 ROCK-METAMORPHISM. 



the beautiful green, ruby, and other colours that especially 

 distinguish it. Rather often may be detected by polarized light 

 septa and strings of this mineral loosely and confusedly interreti- 

 culated with serpentine. In fig. 4, PL III., we have represented 

 a short plate (" chamber-cast " of " Eozoon ") consisting of ser- 

 pentine at one end and peridote with a strong oblique cleavage 

 at the other. Fig. 5, PL III., exhibits peridote forming a portion 

 of another " chamber-cast ; ' and intersected by layers of calcite. 



The interreticulation of peridote and serpentine in the first of 

 the above cases is strongly suggestive of its being due to segre- 

 gative chemical action ; but in the two latter it would seem 

 that the peridote is pseudomorphic after some mineral, partly 

 retaining its crystalline form. The intersecting layers of calcite 

 in the specimen under figure 5 may be accepted as strong 

 presumptive evidence of chemical changes involving the pseudo- 

 morphic development of this carbonate. 



The opinion is pretty general that peridote is in all cases an 

 igneous product. A different view may be entertained by S terry 

 Hunt, who, considering the doctrine of chemical precipitation 

 which he advocates with respect to the origin of the Archsean 

 hemithrenes, ought to regard peridote, at least that form of it 

 which is known to occur in these rocks, as having been formed 

 in the humid or wet way ; for it has come under his notice both 

 in their bedded and " vein "-masses*. 



It has seemingly never struck any one that the frequent 

 intermixture of peridote and serpentine (the latter in bedded 

 and dyke-shaped masses) is strong evidence at least of its being 

 due to chemical alteration. Breithaupt, who had knowledge of 

 masses of serpentine containing peridote, was evidently influ- 

 enced by the prevailing idea, already noticed, in making the 

 suggestion that they are altered examples of the latter mineral 

 that is, of a rock related to what is now called a peridolyte. 

 But as to a rock of this kind being a methylosed dolerite, diorite, 

 or any thing else, the idea to Breithaupt was apparently, as it is 

 to several others of the present day, entirely out of the question. 



The occurrence of peridote in talc-schists seems to have 

 perplexed Bischof ; for he, too, advocated its igneous origin : 

 but being a thorough hydrothermalist as regards the agencies 

 which developed rock -metamorph ism, he was compelled to admit 

 that the case "would seem to indicate that this mineral has 

 * Chemical and Geological Essays, pp. 31 and 210. 



