﻿part 2] 



PICRIT E-TESCHENITE SILL OP LUGAR. 



101 



crystallized in the interspaces left among the earlier constituents. 

 Analcite, however, is more active chemically than silica, and, before 

 its consolidation, finds time to attack and partly to replace some 

 of the other constituents. Thus the analcite is not derived from 

 the alteration of the felspar, as held by some petrographers, but 

 the alteration of the felspars is due to the analcite. 



Olivine is a constant though never abundant constituent in all 

 varieties of the Lugar teschenites. It is invariably replaced by 

 serpentine, which has crept or spread from the original crystal 

 until the form of the latter has been completely obliterated. The 

 serpentinization may be due to juvenile reactions, as suggested by 

 Mr. Bailejr ; but it may also be due, as the migration of the 

 serpentine into the surrounding minerals certainly is, to ordinary 

 weathering. Olivine is most abundant in the coarse teschenites of 

 the Galston type, and almost entirely absent from the nepheline- 

 bearing Cathcart type mentioned above. 



A constant constituent of the Lugar teschenites is ilmenite in 

 peculiar skeletal forms, and invariably associated with a red 

 highly - pleochroic biotite. These ilmenite-biotite groups are 

 most strongly developed in the Cathcart types. The ilmenite 

 presents a variety of skeletal forms, the commonest, perhaps, being 

 an irregularly-shaped, coarsely-reticulate mass. Another form 

 shows a herring-bone structure, with a central axis from which 

 spring rows of thick, parallel, clubbed rods. Biotite fills up the 

 spaces in the skeletal growth. The ilmenite, as a rule, is anterior 

 in crystallization to the pyroxene, and is also frequently enclosed 

 in felspar. Its decomposition gives rise to a grejdsh mass of 

 leucoxene, which is often reticulated with three sets of black bars 

 intersecting at angles of 120°. 



Biotite occurs in three forms ; one, in independent flakes, highly 

 pleochroic from pale straw-yellow to dark reddish brown, is of early 

 consolidation, and is found enclosed in the felspars and analcite. 

 A second form is that described above as occurring in ilmenite- 

 biotite aggregates. The third, which is especially prominent in a 

 Cathcart type from the lower band, is (as described above) an 

 alteration -product of the titanaugite. It occurs as irregular flecks 

 in the interior of the pyroxenes, and also as large, well-formed 

 flakes on their margins. The outer edges of the flakes are quite 

 euhedral, but the interior boundaries with the augite are ragged 

 and indefinite. The biotite is frequently interleaved with chlorite. 

 It often lines a cavity filled with analcite, and serves partly to 

 define the crystallographic form of that mineral. 



Orthoclase is a frequent but variable accessory in the teschenites. 

 As the penultimate constituent to crystallize, it is associated 

 and varies in quantity with the analcite. It is generally much 

 altered, and partly or wholly replaced by analcite. Traces of 

 moire and perthitic structures seem to indicate that it is probably 

 a soda-bearing variety. 



A little altered and turbid nepheline, only recognizable by the 

 shape of the pseudomorphs, is to be seen in some of the rocks, 



