MINERALS IN EUROPEAN BASALTS. 279 



vary in size from a diameter of half an inch to four inches. Near 

 Wiesbaden similar masses of olivine reach a diameter of two feet. In 

 Bohemia the olivine is usually well crystallised. It is generally present 

 in the dolerites of Scotland, but in the North of Ireland and in Iceland 

 there is rarely any trace of olivine, though it is sometimes abundant 

 in definite bands. In polarised light it shows brilliant red and green 

 colours ; it never contains any mineral except magnetite, but often 

 includes glass cavities. In England olivine is usually more or less 

 altered ; and in the midland counties it is always changed into a green 

 substance like serpentine. The alteration begins at the surface of the 

 crystal and extends inward along fine cracks. Serpentinous pseudo- 

 morphs are seen in the doierite of the Glee Hills, of Little Wenlock, 

 Matlock, and Eowley. Olivine is partly converted into hematite in 

 the basalt of Duncarnock in Lanarkshire, and at Bowling, near Dum- 

 barton. It is replaced by calcite in the South Staffordshire Coalfield, 

 and is sometimes replaced by zeolites. 



Bronzite is found in the basalt at Oberwinter on the Rhine, and 

 in the Lohrberg in the Siebengebirge. Iron pyrites and blende are 

 both found in the basalt of Unkel. 



In Antrim and some other localities native iron is found in small 

 particles ; and at Ovifak in Greenland, nickel-bearing iron is plentiful 

 in basalt, and sometimes occurs in very large masses. Titaniferous 

 iron is often found in the Rhine basalts in large visible grains. 



Magnetite is invariably present in British basalts, and, being in 

 octahedrons, shows as an opaque black square, but it is sometimes so 

 clustered that the form cannot be recognised. Specular iron is recog- 

 nised by its blood-red colour, and occurs in thin hexagonal plates. 



Brown mica is present, in some Scotch dolerites, in irregular 

 polygonal plates or strips. In sections parallel to the cleavage plane, 

 biotite is not dichroic, and is always dark between crossed nicols. 

 Sections at right angles to the cleavage plane vary from pale to very 

 dark-brown when the polariser is rotated. Mica is seen in the basalt 

 at Yeitskopf, at Kriifter Ofen, and the Laacher See, and has been 

 noticed near Teplitz and Bilin in Bohemia. 



Apatite is always present in British dolerites in slender hexagonal 

 needles ; it is very common in the felspar and augite, and never 

 encloses any mineral except magnetite. 



Quartz and sanidine have been detected in many localities in 

 North America; and in Europe are found in the basalts of the 

 Siebengebirge. 



The percentage of water varies from 7 \ per cent, in basalts which 

 are rich in zeolites, to nothing in those which are free from zeolites. 

 When the basalt contains nepheline, decomposition of that mineral 

 yields natrolite; but most zeolites in basalt are due to the decom- 

 position of labradorite. 



Agates and chalcedony often fill steam cavities in vesicular basalt. 



The glassy matrix, when present, is seen to be a structureless 

 substance, filling the interstices between crystals. Its structure is 

 sometimes felsitic or even cryptocrystalline. When felsitic it always 





