874 MR JOHN S. FLETT ON 



Chemical analysis was made of a specimen from about six inches from the margin 

 so as to obtain as nearly as possible the average composition. It is given below No. 1, 

 and may be compared with the analyses of similar rocks appended. 



It will be seen that with Nos. 2, 3, and 4, which are typical bostonites, the 

 affinities of the Onston dyke are by no means close. No. 5 is regarded by Professor 

 Brogger as the representative of a distinct group which he names the maenitcs or 

 lime-bostonites. The characteristics he insists on are the large percentage of lime and 

 the preponderance of soda over potash. In the latter respect the Onston dyke is far 

 more near the typical analyses 2, 3, and 4, as it shows a slight excess of potash. 

 Otherwise the two analyses agree pretty well ; but in the Orkney dyke the high 

 percentage of iron, mostly in the ferrous condition, and of magnesia and water, and the 

 lower percentage of alumina, indicate a greater abundance of a chloritic mineral. 



The Camptonite Dykes. 



The dykes included in this series may be porphyritic or non-porphyritic, and vary 

 in coarseness of grain, having often fine chilled edges and a coarser centre. They 

 consist of olivine, augite, brown hornblende, and plagioclase felspar ; the commonest 

 accessory minerals being magnetite, ilmenite, pyrite, apatite, biotite, and orthoclase. 

 Olivine is the commonest porphyritic ingredient ; augite and brown hornblende occur 

 less frequently. The groundmass contains hornblende or augite, or most frequently 

 both of these, with plagioclase felspar, and sometimes orthoclase. The structure is 

 typically panidiomorphic, all the minerals having well-developed crystalline form. The 

 fresh rocks are black or very dark green, rarely showing fluxion structure, and usually 

 with lighter spots, due to the presence of calcite and " ocelli " of felspar. When 

 weathered they are green or rusty brown, and the tendency to spheroidal decomposition 

 is very marked. 



The groundmass of the porphyritic dykes resembles in most respects, as regards 

 composition and structure, the usual type of non-porphyritic camptonite ; and the 

 variety exhibited by the porphyritic minerals gives greater interest to the study of the 

 dykes containing them. I shall select for detailed description the most important 

 dykes with porphyritic structure before considering the much commoner rocks in which 

 this structure is absent. 



The Rennibuster Dykes. — The dyke already mentioned as crossing the west burn at 

 Rennibuster, four miles from Kirkwall, is a remarkably fresh and interesting camptonite. 

 It is 6 feet 6 inches wide, and at the edges finer grained for a few inches, but 

 showing in a compact groundmass crystals of olivine, augite, and hornblende, much 

 altered. Towards the centre it becomes coarser grained, and contains large phenocrysts 

 of hornblende, augite, and olivine. The hornblendes measure in some cases one inch in 

 length, and nearly half an inch in breadth, and are occasionally arranged in a parallel 

 manner, so as to indicate fluxion structure. The augite and olivine are not over a 

 quarter inch in diameter. The weathered material is dark green, the fresh rock is black. 



