OLDEE PEEIDOTITES OF SCOTLAND. 407 



Silica 40-307 



Alumina 12-582 



Ferric oxide 1-809 



Perrons oxide 3-335 



Manganous oxide 0*384 



Lime 7*581 



Magnesia 21-000 



Potash 6-561 



Soda 0-953 



Water 5-738 



100-250 



The small proportion of the iron-oxides and the quantity of lime 

 suggest a resemblance to the mineral of the scy elite. This pale- 

 coloured mica is said to pass into a lustrous brown variety of 

 ordinary biotite, containing 4-913 per cent, of ferric oxide and 19-802 

 of ferrous oxide. 



Nowhere perhaps would it be easy to find a better example of 

 the different changes to which the minerals of igneous rocks are 

 subject. If our interpretation be correct, the scyelite was originally 

 a picrite with well-marked ophitic structure, made up of augite, 

 enclosing and intercrystallized with grains of olivine, with some 

 enstatite, to which was added a considerable quantity of a highly 

 magnesian biotite. 



The first change to which this rock was subjected was clearly due 

 to deep-seated action, and resulted in the conversion of a part, at 

 least, of the augite into diallaj^;^, in the development of tabular and 

 other enclosures in the olivine and enstatite, and in a similar change 

 in the biotite. 



Subsequently to this, and under totally different conditions, a new 

 set of changes was brought about in the rock. The augite was 

 converted into hornblende, the olivine and enstatite into serpentine, 

 and the mica became more or less hydrated, losing in some parts of 

 the mass its physical and optical properties. Yet during all these 

 changes the form and relations of the original minerals were not 

 destroyed, and in the later alterations the structures produced by 

 the first set of changes were so far spared as to admit of our deci- 

 phering the history of this singular rock. 



SUMMAKT OF ReSTTLTS. 



Prom the observations described in the preceding pages, it appears 

 that many rock-forming materials may be made to assume new and 

 unfamiliar aspects by the development of enclosures along certain 

 planes within their crystals. In this way the augites are converted 

 into diallages and pseudo-hypersthenes, and the ferriferous enstatites 

 into bronzites and the varieties to which the name of hypersthene 

 was originally applied ; similarly, olivine passes into a black, opaque, 

 fissile mass, which has frequently been taken for magnetite, and the 

 felspars acquire avanturine and chatoyant characters. Por the process 



2f2 



