400 PROF. J. W. JUDD ON THE TERTIAKY AND 



augite, this mineral remains enclosed in the serpentine, or is con- 

 verted into hornblende. In the latter case we have a rock of the 

 same class as that described by Professor Eonney, from North 

 Wales and the Lake district*, as hornblende-pi crite. Two examples 

 of such altered picrites have been described in the neighbourhood of 

 the Firth of Forth. 



The picrite which forms an eminence on the S.W. of the island 

 in the Firth of Forth was discovered by Mr. Adie, and described 

 first by Dr. A. Geikie f, and subsequently by its discoverer J, The 

 rock may be classed with the picrites, though it sometimes contains 

 a not inconsiderable proportion of felspar. It is an interesting cir- 

 cumstance that the olivine of this rock is sometimes only slightly 

 serpentinized. The augite is sometimes quite intact, but is some- 

 times seen passing into brown hornblende. Some of the hornblende 

 of this rock, however, may be original. The association of augite, 

 hornblende, and biotite in the rock is of the closest kind, but it is 

 difficult to say whether this association, in certain cases, should be 

 interpreted as due to intergrowth or to paramorphic changes, I 

 have been kindly supplied by Mr. T. Waller, of Birmingham, with 

 the following analysis of this beautiful rock: — 



Silica 37-8 



Alumina 9*7 



Ferric oxide 3*4 



Ferrous oxide 7*0 



Lime 4-1 



Magnesia 22-9 



Soda (with trace of potash) .... 0-8 



Loss on ignition 14-0 



99-7 



As leucoxene occurs among the alteration -products in the rock, 

 titanic acid is certainly present ; it was not, however, specially 

 determined in the analj^sis, and it has therefore been included in 

 part with the silica and in part with the alumina. 



The specific gravity of this rock is 2-81. 



The picrite of Blackburn, near Bathgate §, which has also been 

 described by Dr. A. Geikie, resembles that of Inchcolm in its 

 mineralogical constitution, but is of especial interest to geologists 

 on two grounds. If the interpretation given of the relations of this 

 rock be a correct one, it affords an example of an ultrabasic rock 

 occurring as a lava-stream, and at the same time illustrates the 

 possibility of the heavier minerals in a lava sinking to the bottom 

 of it, so as to cause the lower portions of the stream to be of more 

 basic character than its upper part. 



There is one of the older peridotites of Scotland, however, which 



* Quart. Journ. Geol. Soc. vol. xxxvii. (1881) p. 137, and xxxix. (1883) p. 254. 

 t Trans. Roy. Soc. Edinb. vol. xxix. (1880) pp. 507, 508. 

 J Cole's Studies in Microscopical Science, vol. i. (1882) p. 45. 

 § Trans. Roy. Soc. Edinb. vol. xxix. (1880) pp. 504-507. 



