004 MR JOHN S. FLETT ON 



XI. L. V. Pirsson*, "On the Monchiquites or Analcite Group of Igneous Rocks," Tlie Journ. of Geology, 

 vol. iv., 1896, p. 679. 



XII. W. LiiNdgren, "Eruptive Rocks from Montana," Proceedings of the California Academy of Science, 

 1890, vol. iii. p. 39. 



XIII. Whitman Cross, "An Analcite-basalt from Colorado," Tlie Journ. of Geology, vol. v., 1897, 

 p. 684. 



XIV. W. H. Weed and L. V. Pirsson, "The Geology of the Castle Mountain Mining District," 

 Bulletin of tlie United States Geological Survey, No. 139, 1896. 



XV. H. Rosenbusch, Mikroskopische Physiographie der massigen Gesteine, Ed. III., 1896. 



XVI. M. Hunter and H. Rosenbusch, "Uber Monchiquit, ein camptonitisches Ganggestein aus der 

 Gefolgschaft der Elaeolitsyenite," Tschermak's Mineral ogische Mittheilungen, vol. xi. 26, 1890. 



XVII. F. Berwerth, " Ueber Alnbit von Alno," Annalen des k. k. naturhistorischen Hofmuseums, Wien, 

 1893, Band VIII. p. 441. 



XVIII. Frank D. Adams, "On a Melilite-bearing Rock (Alnbite) from Ste. Anne de Bellevue, near 

 Montreal, Canada," American Journ. of Science, vol. xliii., 1892, p. 269. 



XIX. H. Rosenbusch, Elemente der Gesteinslehre, Stuttgart, 1898. 



XX. Hillebrand, "Chemical Analyses of Rocks," Bulletin of the United States Geological Survey, 

 No. 148. Washington, 1897. 



XXI. Williams, "Igneous Rocks," Arkansas Geological Survey, Annual Report, 1890, vol. ii. 



XXII. Merrill, G. P., Rocks, Rock-Weathering, and Soils, 1897. 



EXPLANATION OF THE PLATES. 



Plate I. 



Fig. 1. Photomicrograph of the Bostonite of Onston Ness, Stennis, Orkney — magnified about 20 

 diameters. The elongated sections of felspar, which is both orthoclase and plagioclase, are arranged in a 

 fluidal manner. The black spots are magnetite and ilmenite weathering to leucoxene. Chlorite and other 

 secondary products in small quantity occupy the interspaces between the felspars. From near the margin of 

 the dyke. 



Fig. 2. The Bostonite of Onston Ness, Stennis, Orkney, photomicrograph between crossed nicols — mag- 

 nified about 30 diameters. The section is taken from the centre of the dyke and shows a large Carlsbad 

 twinned crystal of anorthoclase, filled with decomposed glass cavities, and surrounded by a groundmass not 

 markedly fluidal, of simply twinned felspars. 



Fig. 3. Camptonite, Rennibuster Burn, Orkney — magnified about 20 diameters. Large idiomorphic 

 phenocrysts of a pale augite, surrounded by a decomposed zone, lie in a groundmass of idiomorphic hornblende 

 prisms and clear plagioclase. On the right margin a segment of an ocellus rich in felspar, radiately disposed, 

 with, at the upper margin, an area of calcite occupying a miarolitic cavity. 



Fig. 4. Camptonite, Scabra Head, Rousay, Orkney — magnified 20 diameters. A more fine grained 

 rock than the preceding, but with a similar groundmass. The phenocrysts are augite, idiomorphic and pale 

 in colour, and dark hornblende slightly corroded and showing zones of different shade. 



Fig. 5. Camptonite, Stromness, Orkney — magnified 26 diameters. At the bottom two phenocrysts of a 

 violet augite. The groundmass consists of hornblende and augite, intergrown and idiomorphic, surrounded 

 by clear plagioclase. In the upper part two felspathic ocelli. 



Fig. 6. Camptonite, North Galton, Sandwich, Orkney. From the centre of the dyke — magnified 35 

 diameters. Large augite phenocrysts are seen in the margins below and above. On their surfaces lie lath- 

 shaped felspars which, in the centre of the photograph, are enclosed by allotriomorphic brown hornblende, in 

 an ophitic fashion. 



Plate II. 



Fig. 1. Camptonite, West Dyke of Hoxa, South Ronaldshay, Orkney — magnified 20 diameters. The 

 phenocrysts are augite, idiomorphic, almost free from enclosures, and pale in colour, and hornblende (almost 

 in the centre of the figure), with a dark brown laminated centre and a clear peripheral zone. In the upper 

 part are three crystals, the centre of which consists of an augite-hornblende intergrowth, dark with grains of 

 magnetite, and showing only rude traces of crystalline boundaries, surrounded by hornblende which may con- 

 sist of two zones, the inner paler in colour than the outer. Just below the centre of the figure is a similar 

 crystal, the upper margin of which is formed by a thin rim of hornblende, while the remainder of the margin 

 is a paler augite. Details of the groundmass are not shown, but it is rich in hornblende and augite, and con- 

 tains comparatively little felspar. 



