354 PEOr. J. W. JTIDD ON THE TEETIAET AI^D 



50. On the Teetiaet and Oldee Peexdotites of Scotlaot). By 



Prof. JoHi^ W. JuDD, F.E.S., Sec. G.S. (Eead February 11, 



1885.) 



[Plates X.-XIII.] 

 Introduction. 

 Part I.' — The Tertiary Peridotites and Allied Eocks. 



§ 1. Eelations of these Eocks to the other Eruptive Masses of the 



Western Isles of Scotland. 

 § 2. Microscopic Structures of the Eocks. 

 § 3. Minerals of which the Eocks are built up. 

 § 4. The Changes which these Minerals have undergone at great depths 



from the surface. 

 § 5. Nature and Origin of the Changes which have taken place in the 



Minerals of deep-seated Plutonic Eocks (" Schillerization"). 

 § 6. The Agency by which the Schillerization of Minerals has been pro- 

 duced. 

 § 7. Varieties of the Tertiary Ultra-basic Eocks. 

 Part II. — The Paleozoic Peridotites and Allied Eocks. 

 § 1. Alteration of the Minerals in the Paleozoic Peridotites. 

 § 2. Varieties of the Palaeozoic Peridotites. 

 § 3. The Scyelite (altered mica-hornblende-picrite) of Caithness. 

 Summary op Eesults. 



Inteodtictigis^. 



Those rocks which contain an excessive proportion of the bases, 

 especially magnesia and ferrous oxide, and are therefore composed 

 largely of unisilicates, may be conveniently classed as ultra-basic 

 rocks. Such rocks constitute a small but highly interesting group, 

 which is characterized as follows : — They have a very low percentage 

 of silica, ranging generally from 35 to 45, with a high specific 

 gravity, varying from 3 to 4 ; while the ferro-magnesian silicates, 

 olivine and enstatite, enter largely into their coustitution. Felspar 

 is often altogether absent from these rocks, and, when present, 

 appears to be always represented by the basic species anorthite, or 

 one approaching in composition to that type. 



Many of these ultra-basic rocks may be very conveniently grouped, 

 as Professor Eosenbusch proposes, under the name of " peridotites ;" 

 rocks, that is, in which the unisilicate, olivine, forms the prevailing 

 constituent. 



There are other ultra-basic rocks, however, which contain a con- 

 siderable proportion of the bisilicates, and these form a link between 

 true peridotites and the ordinary basic rocks. Among these may be 

 noticed the picrites, in which olivine is united with a considerable 

 proportion of augite, hornblende, or biotite, anorthite-augite rock 

 (eucrite)*, anorthite-hornblende rock (corsite), and anorthite- 

 olivine rock (troctolite or forellenstein). 



[ * It is possible that teschenite must be grouped with those rocks which lie 

 on the border line between the basic and the ultra-basic groups. See Eohrbach, 

 Tschermak's Min. und Petr. Mittheil. vol. vii. (1885) p. 1.1 



