406 PEOr. J. W. JTJBD ON THE TEETIAEY AND 



of mica. The hornblende occupies precisely the same relation to 

 these minerals as the augite does to the olivine in the peridotites of 

 the Shiant Isles. The structure of the scyelite is therefore distinctly 

 ophitic. 



The study of this rock leads to the conclusion that, in spite of 

 the clear and fresh appearance of most of the minerals of which it 

 is made up, they are all of secondary origin. 



The hornblende is a paramorph after augite, some at least of 

 which had been previously converted into diallage. Yery similar 

 hornblende occurs in some examples of the picrite of Schriesheim, 

 and this rock exhibits, as Professor Bonney observes, every gradation 

 from augite to hornblende *. 



The serpentine of the rock clearly replaces olivine, and in some 

 cases probably enstatite also, as is indicated by the peculiar nature 

 of the enclosures, which often persist and retain their form after the 

 hydration of the enclosing mineral. 



Whatever the micaceous mineral should now be called, it appears 

 to have been originally a highly magnesian biotite, that is to say 

 an approximately uniaxial mica. Its pale colour and feeble 

 pleochroism appear to be clearly related to the low percentage of 

 iron which it contains. 



Another peculiarity in this mica is probably the replacement of much 

 of the alkalies by basic water. How far these peculiarities of the 

 mineral are original, and how far they are of secondary origin, it 

 may be difficult to determine. The abundant tabular enclosures in 

 the mineral point to the conclusion that Schillerization, or the dis- 

 solving-out of the iron and its collection into the hollows of negative 

 crystals, has gone on to a considerable extent. It was probably at 

 the first a biotite, very rich in magnesia and poor in iron, with the 

 potash largely replaced by basic water ; but some of the iron and 

 the alkalies have not improbably been removed during the process of 

 Schillerization. 



That such biotites with a low percentage of iron not unfrequently 

 occur in the ultra-basic rocks is highly probable. I have found a 

 mica similar to that of the scyelite in several altered peridotites, 

 among others some varieties from Schriesheim. The mineral may 

 easily be mistaken for an altered enstatite, from its optical properties. 



Dr. Heddle has analyzed a mineral occurring in the serpentine of 

 Mmtown, Glen Urquhart t, which appears to be analogous to the 

 biotite of the scyelite. The mineral is stated to be pale green or 

 nearly white in colour, to have a specific gravity of 2'781, and to be 

 associated with large pale crystals of hornblende. 



The analysis is as follows : — 



* Quart. Journ. Greol. Soc. vol. xxsix. (1883) p. 256. [Professor Cohen has 

 recently pointed out that the mineral formerly taken for diallage in tJie 

 Schriesheim picrite is reaUy hornblende (Neues Jahrb. fiir Min. 1885, vol. i. 

 p. 242).] 



t Trans. Eoy. Soc. Edinb. vol. xxix. (1880) p. 18. 



