628 PROF. G. H. F. ULRICH ON THE 



is dusky, sometimes nearly opaque. With the olivine the case is 

 difiPerent. While in parts of the sections it is traversed by an 

 irregular network of veins of serpentine, in other parts, of about 

 similar extent, such veins are scarce, smaller, or quite absent, and 

 there are only noticeable scattered patches with more pronounced 

 cracks and of slightly impaired transparency, caused by a minute 

 dust or granulation, and which show mostly aggregate polarization 

 between crossed Nicols ; the brilliancy of the polarization colours is, 

 however, quite unaffected. Regarding the serpentine of the veins, 

 it is different from that formed from the enstatite, being perfectly 

 transparent and nearly colourless throughout ; and its behaviour, as 

 well as the structure of the veins in polarized light (between crossed 

 Nieols with rotation of the stage), accord perfectly with the descrip- 

 tions given by Prof. Bonney in his paper on the serpentine of the 

 Lizard district, Cornwall *, and therefore need not be detailed here. 

 Only I may mention that in some of the veins the arrangement of a 

 granular dark layer in the centre and a transversely-fibrous layer 

 along each side is very regular, reminding us of the symmetrically 

 banded structure observed in some ore-lodes ; also that in parts of 

 some veins the fibrous bodies are more or less regularly radiating 

 (star-like) from dark ferruginous granules. Inclusions of grains of 

 chromite, &c., in the olivine and enstatite are, according to the pre- 

 pared sections, very scarce indeed ; one pretty large section shows 

 not a single grain, and none of the others contain more than three 

 or four. The specific gravity of the rock, as determined on dif- 

 ferent specimens, ranges from 2*81 to 3*13 ; and regarding its 

 solubility in HCl, some determinations by Mr. Thomas Batement 

 gave the following results : — 



No. 1. No. 2. No. 3. 



Soluble 80-08 7, 83-73% 84-11 7« 



Insoluble .... 19-92 „ 16-27 „ 15-89 „ 



100-00 100-00 100-00 



The soluble portions here evidently represent olivine and serpen- 

 tine ; the insoluble enstatite, with a small percentage of chromite, &c. 

 Judging from what the sections show, the relative proportions of 

 olivine and enstatite in the unaltered original rock may be taken to 

 have been as 2 : 1, or at the outside 3:1, say as 70 7o - ^^ 7o 5 there- 

 fore the above results would indicate that in the present rock nearly 

 one-half of the enstatite has been converted into serpentine. As 

 already mentioned when describing the thin sections, it is rather 

 questionable whether the olivine has suffered to a like extent. 



The second variety of rock from the Olivine Eange is of medium- 

 grained, granitoid texture, dark greyish-green colour, and very tough. 

 The amount of outstanding grains of enstatite on the brown- 

 weathered surface of the rock points to this mineral predominating 

 over olivine, and this is confirmed by examination of the several 



* Quart. Journ. Geol. See. vol. xxxiii. 1877, pp. 907 & 916. 



