PLATE XXX, 



FIGS. 1 and 2. 

 ANDESITIC DOLEKITE. 



BAH WICK QUARRY, NEAR PRESTON, ON THE TEES. 



Magnified 25 diameters. Ordinary light. 



Fig. 1 represents a section of one of the large porphyritic felspars (labra- 

 dorite). This felspar contains large inclusions of the ground-mass and grains of 

 augite (7). 



Fig. 2 represents the microscopic structure of the ground-mass in which the 

 felspars lie embedded. The substances present arc plagioclase, augite, magnetite 

 (5), pyrites (19) and interstitial matter. The augite occurs in small, almost 

 colourless granules which arc not well seen in the figure. The pyrite is drawn 

 as it appears in transmitted light ; by reflected light it shows the characteristic 

 yellow metallic lustre. The brown interstitial matter gives a vague reaction 

 with crossed nicols. It contains minute and often feathery microlites of felspar. 

 The skeleton felspars represented in Fig. 7, page 15, occur in this slide. 



The dyke is almost certainly of Tertiary age. It cuts Oolitic rocks and 

 alters them. 



