﻿694 PROF. T. G. BONNEY AND MISS C. RAISIN ON THE [Nov. 1905, 



Lave apparently forced their way. Their structure is obscure, but 

 they seem to be formed of fibres lying in the general direction of 

 the cleavages. Their polarization-tints range from the white to 

 the paler yellow of the first order, being perhaps a little lower than 

 those of the strings in the altered olivine. We also find, at longer 

 intervals, and with less regular direction, strings which probably 

 mark the position of a very imperfect basal cleavage. The parts of 

 the altered crystal between this network often show a minute 

 fibrous structure, sometimes accentuated by interrupted lines of 

 opacite. This yields low polarization-tints, but the most structure- 

 less parts produce hardly any effect on polarized light. Certain of 

 the Lizard serpentines — those from Lower Pradanack, from south 

 of Mullion, from the pit above Carnbarrow, from the mass below 

 Kildown-Point Quarry, and from the northernmost outcrops on the 

 Helston road — have all a general resemblance to the Eauenthal 

 rock, though the olivine is occasionally, and the hornblende 

 generally, better preserved. In the last-named Cornish example, 

 however, one or two specimens of the latter mineral correspond 

 exactly with those in the Yosges rock. 



IV. Orthorhombic Pyroxenes. 



These ferromagnesian silicates form a series resembling that 

 of the olivines. Tschermak distinguishes those in which the iron 

 lies between 1 and rather more than 20 per cent., the magnesia 

 generally ranging from 40 to 30 per cent., into enstatites and 

 bronzites ; and the remainder, in which the iron is above 30 per cent, 

 and the magnesia correspondingly depressed, into hypersthenes and 

 amblystegites. The first symptom of change, and it appears to be 

 more readily developed in the former two, is the replacement of the 

 usual cleavages parallel to the prism-faces by one parallel to a 

 pinacoid. Varieties in which the percentage of silica is rather 

 high and that of ferrous oxide is generally low have been 

 called diaclasite, but in the absence of proof of chemical 

 composition we shall apply the name bastite 1 to all forms 

 occurring in serpentines. Signs of this change in the cleavage may 

 be perceived when the olivine in a slice looks quite fresh, but 

 conversion into serpentine fakes effect more rapidly in the latter 

 mineral. 2 Enstatite also alters into talc, but apparently with less 

 ease, the reason for which will be noticed below. 



The polarization-tints of enstatite (which usually are not so high 

 as those of augite) are lowered by change through bastite to 

 serpentine, and the cleavage -planes become, as it were, soldered 

 together ; but traces of them can be seen under the microscope, 

 which, with the different kind of serpentinization, generally enable 



1 The optic axial plane, it will be remembered, is parallel in enstatite to the 

 plane of easy cleavage, in bastite perpendicular to it. 



2 To this rule I have found two or three exceptions — for example, a 

 serpentine from Cam Sparnack (Lizard), in which the bastite is replaced by a 

 kind of steatite, and yet there is a fair amount of residual olivine. [T. G. B.] 



