PLATE xin. 



FIG. I. 

 FELSPAR - AUGITE - HORNBLENDE ROCK, 



FORMING A VKIN IN MICACEOUS DOLEKITE. 



DENISTON HILL, GLASGOW. 

 Magnified 50 diameters. Ordinary light. 



This figure shows felspar (10), augite (7), hornblende (8) and magnetite. 

 The felspar gives broad lath-shaped sections and is more or less changed to a brown 

 granular decomposition product. The augite exhibits a pale violet tint, and occurs in 

 fairly well-formed crystals. It is changed to chlorite along the margins in places. It 

 occurs as inclusions in the felspar and was evidently formed before the latter mineral. 



The hornblende is the deep brown " basaltic " variety, sometimes known as 

 basaltine. It occurs in well-formed crystals in other portions of the slide, and shows 

 the characteristic pleochroism (, yellowish brown ; /3 and 7 a rich clove brown). This 

 rock has been described by Mr. ALLPORT (Q.J.G.S., Vol. XXX, p. 5(51). 



FIG. II. 

 ENSTATITE - DOLERITE. 



PORTION OF THE WHIN SILL, MIDDLETON. 

 Magnified 50 diameters. Ordinary light. 



The minerals represented are augite (7), plagioclase (10), enstatite (2), mica (9), 

 and magnetite or ilmenite. 



The augite is nearly colourless. It occurs in the form of irregular grains and 

 granular aggregates. The felspars penetrate the augite ; but the rock does not show 

 the ophitic texture in any marked degree because the latter mineral does not occur 

 in the form of large irregular plates (compare fig. 1, plate X.) 



The felspar occurs in lath-shaped sections which are as a rule rather short in 

 proportion to their length, and also as irregular colourless plates. The analysis of the 

 isolated felspar substance suggests the presence of andesine, or at any rate a felspar 

 of more acid composition than labradorite. 



In the upper part of the figure is a longitudinal section of a colourless rhombic 

 pyroxene (enstatite). It gives the characteristic low tint between crossed nicols, and 

 extinguishes parallel to the length of the section. The alteration of this mineral to 

 a green fibrous substance (bastite) having the axes of the fibres parallel with the 

 vertical axis of the original mineral is highly characteristic of the rhombic pyroxenes. 



Two irregular flakes of a brown mica are represented in the figure. The only 

 other mineral in addition to the magnetite or ilmenite which requires notice is the one 

 occurring in long colourless acicular microlites. This is probably apatite. 



