PLATE XXXVI. 



FIG. 1. 

 ENSTATITE-AUGITE-ANDESITE. 



HALF-A-MILE ABOVE WINDY HAUGH ON THE COQUET ; CHEVIOT DISTRICT. 



Magnified 25 diameters. Ordinary light. 



The porphyritic constituents are labradorite (10), enstatite (2), and 

 augite (7). 



The labradorite occurs in detached crystals and in groups. One group 

 containing two augite-grains is represented in the lower part of the figure. The 

 internal boundaries of the individuals forming a group are irregular ; the external 

 boundaries are formed by crystalline faces. The individual grains in a group are, 

 therefore, idiomorphic externally and allotriomorphic internally. 



The ground-mass is composed of a pale brown glass, small felspars, specks 

 of opacite and minute hexagonal plates of hematite (eisen-glimmer). The 

 latter, however, cannot be seen with the magnifying power employed. The red 

 vein in the upper part of the figure is formed of opal, deeply stained with 

 ferric oxide. 



FIG. 2. 

 ENSTATITE-ANDESITE. 



THE ALWIN ; CHEVIOT DISTRICT. 



Magnified 50 diameters. Ordinary light. 



The constituents of this rock belong to three well marked periods. Those of 

 the first period are labradorite (10), and enstatite (2). They constitute the 

 porphyritic elements and are the only definitely recognizable minerals in the 

 figure. A good longitudinal section of eustatite, with the characteristic 

 terminations formed by wo flat domes, is seen somewhat below the centre of 

 the figure. 



The constituents of the grouudmass belong to the second period. They 

 comprise oligoclase-microlites (giving straight or nearly straight extinction) 

 magnetite grains and isotropic glass. 



The brown colouring matter (ferric oxide) developed in certain places and 

 especially along cracks is an alteration product and must be referred to the 

 third period. 



