892 



MR JOHN S. FLETT ON 



trend, and seem to have a marked tendency to branching. There can be no doubt 

 that they are, both in geological origin and in petrographical characters, a closely 

 related series. 





1. 



2. 



3. 



Si0. 2 

 Ti0 2 



ALA 



Fe,0 3 



FeO 



MnO 



CaO 



MgO 



Na.,0 



K 2 



H 2 



Spec. Grav. 



40-65 

 4-52 



17-12 

 4-26 

 5-53 

 0-34 



12-88 



9-96 



1-74 



2-80 



•36 



40-79 



17-36 

 383 



15-04 

 0-30 



10-83 

 6-97 



I 417 



0-71 



39-88 



4-86 



14-83 



| 12-60 



12-68 

 12-27 



3-39 



100-16 

 3-153 



100-00 



100-51 



1. Hornblende from hornblende-monchiquite, Copinshay, Orkney. 



2. Analyses of hornblende from camptonite. Dixville North, N.H., U.S.A. (cited from XIX., p. 234). 



3. Average of ten analyses of basaltic hornblende. Brb'gger (VI., p. 29), after Schneider's analyses. 



One of the most interesting occurs in a small headland about 400 yards north of the 

 farmhouse of Rennibuster, four miles from Kirkwall. It is 5 feet broad, and in the 

 hand specimen is a dark green, almost black rock, with fine glancing scales of biotite 

 and vesicles filled with calcite and pale-coloured zeolites. The weathered material is 

 dark rusty brown. 



Under the microscope, olivine proves to be abundant in phenocrysts, rather uniformly 

 of small size. It is entirely decomposed into pale green, sometimes almost colourless, 

 serpentine accompanied by magnetite, or into calcite and limonite. It encloses magnetite, 

 and perofskite in small dark brown octahedra. Augite in large phenocrysts is compara- 

 tively scarce, and usually idiomorphic, with short, broad, longitudinal, and octagonal 

 transverse sections. The centre is almost quite colourless, the margins purplish-brown. 

 It is twinned, often repeatedly, on 100, and has the usual cleavage. It shows no 

 decomposition. The enclosures are apatite, magnetite, and perofskite. 



The most striking porphyritic mineral is biotite, in large irregular plates. Its 

 pleochroism is intense, a = pale yellow, fr and % = reddish-brown, absorption - 1 = b>;i. 

 A very narrow zone of intenser brown surrounds a large corroded nucleus, and may 

 occasionally show a tendency to idiomorphism by the presence of angular corners 

 recalling hexagonal outlines. It has a greater absorption than the central biotite for all 



