620 ME. H. H. ARNOLD-BEMROSE OK THE MICROSCOPICAL [Nov. 1 894, 



should thus have the change proceeding from the cracks of the 

 original olivine, in a direction parallel to the length of the crystal, 

 and not normal to the cracks, as in the case of the alteration to 

 serpentine. The whole olivine-crystal is at length replaced by this 

 mineral, which is yellow along the cracks and green in the remaining 

 parts. More perfect cleavage is developed, the iron becomes oxidized, 

 colouring the mineral red instead of green and yellow, and we have 

 the mica-like mineral as a resulting product, the cleavage-planes 

 being parallel to the macropinacoid and the plane of the optic axes 

 often perpendicular to (001). 



The mineral described by Prof. Iddings is evidently a pseudomorph 

 of olivine. Lawson considers that iddingsite is not a pseudomorph 

 of olivine, that it is separated from olivine by its chemical, optical, 

 and physical characters, and states that no trace of olivine or its 

 ordinary decomposition-products has been detected in his thin 

 sections. In the case of the Derbyshire mineral the same remarks 

 would hold good, if we only had specimens from Potluck and Upper- 

 wood to deal with (even in these outcrops, if the rock were quarried, 

 fresh olivine might be found in less altered portions). But we 

 have the same mineral associated with olivine in two other outcrops. 

 It may be possible, therefore, that iddingsite is a pseudomorph or 

 replacement of olivine, but that the replacement has extended 

 throughout the whole of the rocks in question. 



In the absence of a quantitative analysis of the Potluck pseudo- 

 morph, we cannot be certain whether it is a mica. So far as the 

 present evidence goes, it would appear to be a mineral allied to 

 biotite or to clintonite, and may be the same as that which Prof, 

 llenard considered from its optical properties to be biotite. At 

 present I prefer to call it a ' mica-like mineral replacing olivine.' 



4. Rhombic Pyroxene. 



This mineral resembles that in the lava of Eycott Hill described 

 by Prof. Bonney as altered enstatite. 1 



Mr. Teall kindly lent me several thin sections of the Eycott Hill 

 rock for comparison. The pyroxene in the Derbyshire rock is pale 

 green or yellow in transmitted light ; longitudinal sections are 

 more or less pleochroic, the absorption being greatest when the 

 traces of cleavage and the length of the section are parallel to the 

 short axis of the polarizer. Extinction always takes place parallel 

 to the length of the section. 



The mineral occurs mainly in three localities. A specimen from 

 Staden Low, outcrop 29, contains both individual crystals and 

 groups of crystals. The largest crystal measures 1x4 millim. It 

 has traces of cleavage parallel to its length, behaves as above stated, 

 and gives pale yellow polarization-colours between crossed nicols. 

 One individual of a group of about 12 gives an eight-sided section. 

 The angles between 3 consecutive pairs of adjacent sides are 134°, 



1 Geol. Mag. 1885, pp. 76-80. 



