G. W. Tyrrell — Bekinkinite of Barshaw, Renfrewshire. 307 



and essexite-dolerite from the Ayrshire province 1 and to the crinanites 

 of Argyllshire and Arran. 2 It is worthy of note that the essexite- 

 dolerites of Ayrshire are intimately associated with certain highly 

 analcitic types. 



MlNEEALOGICAL DESCRIPTION. 



All varieties of the Barshaw rocks described later contain the 

 same set of minerals, and differ only in the relative proportions of 

 each. It is therefore convenient to describe the mineral constituents 

 together. These minerals are titanaugite, barkevikite, aegirine, 

 ilmenite, olivine, apatite; nepheline, analcite, plagioclase, orthoclase; 

 with various decomposition-products. 



The titanaugite is a deeply-coloured variety, with a relatively 

 intense pleochroism, ranging from a clear pale brown to a deep 

 maroon or purple-madder tint. The colours are very patchy in their 

 distribution. Sometimes they form regular zoning parallel with the 

 margin of the crystal, sometimes a good hour-glass structure. More 

 often the tints have no regular distribution ; but the different part* 

 are generally separated by a sharp distinct line, and do not fade on& 

 into the other. Occasionally there is a narrow exterior zone of green 

 aegirine-augite. The double refraction is high, with strong dispersion 

 of the bisectrices, so that sections parallel to the symmetry-plane do 

 .not give complete extinction in white light. It is euhedral to all 

 minerals save olivine and plagioclase. It is often irregularly indented 

 by the terminations of the latter crystals. 



The amphibole is a deep red-brown variety, referred to barkevikite. 

 It is highly pleochroic, the extremes of colour being clear pale yellow 

 and a deep red-brown. The latter has often a faint purplish shade. 

 There is frequently a very narrow exterior zone of greenish or bluish 

 material (arfvedsonite ?). Many sections have a small irregular core 

 or central interlamellation of titanaugite in parallel intergrowth, and 

 these are undoubtedly primary crystals which have grown upon 

 a core of augite. Most of the barkevikite is primary, and in some 

 of the rocks it is unaccompanied by titanaugite. Where, however, 

 the latter is in excess, as in the melanocratic type, the amphibole 

 forms a more or less broad exterior margin to the augite crystal, 

 having an outer crystalline form, but with extremely ragged, irregular, 

 interior margins against the pyroxene. Here it is clearly an out- 

 growth upon the pyroxene, not due to weathering, but to molecular 

 instability of the pyroxene during or after crystallization. The 

 barkevikite may enclose the terminations of felspar laths of early 

 crystallization, but both it and the pyroxene is occasionally found 

 enclosed within the later, highly-zonal plagioclase. 



Small crystals of deep green cegirine occur enclosed in the analcite 

 and nepheline. 



Ilmenite occurs in skeletal crystals which frequently have an 

 ■ external euhedral form. It is always partially altered to leucoxene 

 or granular sphene, and frequently presents a barred appearance ; or 

 unaltered ilmenite is left as an external zone around a core of 



1 Tyrrell, Geol. Mag., Dec. V, Vol. IX, p. 124, 1912. 



2 Tyrrell, Geol. Mag., Dec. VI, Vol. X, p. 307, 1913. 



