A Hornblende-Picrite from Valcm. 203 



On examination with the microscope the olivine is seen to be 

 fresh-looking, forming the usual rounded grains with reticulate 

 structure, where alteration is just beginning. At one part of the 

 slice, around the olivine and separated from it by a sharp, sometimes 

 crystallographic boundary, is clear unaltered hornblende, often 

 extending as a large crystal enclosing many grains. It is brownish, 

 dichroic, exhibits low brownish tints with crossed nicols, and well- 

 marked cleavages along which deeper-coloured rhomb-shaped 

 inclusions are often ranged — examples of Professor Judd's 

 schillerization enclosures. These apparently have a tendency to be 

 formed along an internal zone of the crystal. 



At another part of the slide, the olivine is imbedded within 

 a mineral generally colourless, much of which is undoubtedly 

 augite.^ Sometimes, however, a faint dichroism, a brownish colour, 

 and characteristic lattice cleavage are exhibited, and it seems as if 

 the augite is changed to hornblende.^ At one part a bordering 

 mineral similarly related to the olivine, but having a close fibrous 

 actinolitic structure, like the pilite of Becke, may be a further 

 modification of the original pyroxene. 



The third important constituent (forming the white specks seen 

 macroscopically) is a mineral, now mostly a dusty-looking opaque 

 substance, but often ch'anged at least at the exterior to serpentinous 

 or some non-polarizing aggregate. Eemains of one good cleavage and 

 sometimes of a cross cleavage are well marked, and the mineral 

 seems to be decomposing bastite or steatite. The rhombic pyroxene 

 apparently developed late, filling up the interspaces between the 

 hornblende. At one side of the slice a large crystal of fresh-looking 

 enstatite^ surrounds the olivine, and is intercrystallized with some 

 of the bordering hornblende and even with some of the decomposed 

 bastite. 



It thus seems as if the monoclinic was the first of the pyroxenes, 

 starting and forming on the edge of pre-existent olivine, but 

 was in some cases speedily followed by the orthorhombic pyroxene. 

 Thus, occasionally a large crystal of augite or hornblende occurs, 

 lustre-mottled by the olivine, but the extended crystal may be 

 represented only by rims, and bastite may fill up much of the 

 space.* This at places gives to the latter the aspect of an inclusion, 

 but its form shows that it followed the olivine and was not a 

 contemporaneous formation. 



The spinellid is pleonaste of a rather bright green colour, often 

 in partly rounded grains, occurring sometimes within hornblende, 



1 It is usually not dichroic, exhibits well-marked cleavage planes, and the high 

 extinction angle of augite. 



^ As described by Professor Bonney, " On the so-called Diorite of Little Knott 

 (Cumberland), with further remarks on the Occurrence of Picrites in Wales": 

 Q.J.G.S. 1885, vol. xli, p. 520. 



3 The enstatite has close parallel cleavages which almost seem to form a repeated 

 twinning, but this (examined with a high power) is seen to be an effect of a large 

 number of elongated vesicular-looking enclosures, which are probably negative 

 crystals formed along gliding planes. Cf. Prof. Judd, Q.J.G.S. 1885, vol. xli, p. 354. 



* This may possibly indicate a local deficiency of lime in the magma, a thing not 

 surprising, as peridotites are often ill-mixed rocks. 



