ORIGIN OF MINERALS CHARACTERISTIC OF OPHITES ETC, 71 



duller, and their variation is somewhat different *. These differ- 

 ences, however, are no more than what may be observed in most 

 minerals assumed to be peridote. It may also be mentioned 

 that, considering the approximate agreement between peridote 

 and augite (woocut fig. 2) in their cross sections, Dr. Hull was 

 justified in assuming the crystal to be peridote : for the same 

 reason we see no impropriety in taking it to be augite ; but the 

 sectional difference they exhibit is not to be overlooked. 



Still, although so much can be said in favour of the identifica- 

 tion made by Hull, it is extremely doubtful to us that it is a 

 correct one, inasmuch as the prismatic cleavage, so well displayed 

 in the crystal under notice, instead of being right-angled in its 

 intersections or parallel with the sides of a rectangular prism, as 

 required for peridote, is rhombic, precisely like that of augite. 

 We are therefore inclined to the conviction that this example was 

 originally a crystal of augite, which has undergone a chemical 

 change into a substance resembling that of peridote, or, as we 

 prefer to say, which has become peridotized. 



The presence of peridote in certain rocks may be satisfactorily 

 explained in accordance with the views that have been advanced ; 

 for, admitting that crystals of hornblende and augite can be 

 pseudomorphosed into peridote, there is no reason why a crys- 

 talline rock, whether xerothermal, methylosed, or volcanic, 

 which contains these minerals, may not become more or less 

 peridotized. It is in this light that we regard dunyte, picryte, 

 Iherzolyte, ossipyte, olivenyte, and others of their class, also 

 many of the doleritic dykes described by Allport f. We are 

 therefore opposed to accepting the presence of peridote in any 

 one of the above-named examples as evidence that a rock of the 

 kind is in its original condition, but rather as proving that it is 

 more or less a methylotic product. 



* When the prisms are parallel the colour is pale yellow, at 45 pale 

 purple, at 90 (cross prisms) deep purple, at 135 pale pink, returning to 

 pale yellow: there is no brilliant sap-green, nor ruby. The structure is 

 granular. 



t The remarkable doleritic rock at Carmoney Hill, co. Antrim (the matrix 

 of the new mineral, hullite, Hardman), is so charged with peridote, as first 

 made known by Prof. Hull (Proc. Royal Irish Acad. 2nd ser. vol. iii. 

 (Science), pp. 166, 167), that we are disposed to consider it another ex- 

 ample of the kind. 



