PLATE xu. 



DOLERITE. 



TYNEMOUTH, NORTHUMBERLAND. 

 Magnified 25 diameter*. Fig. I., ordinary light ; Fig. II., with crossed nicols. 



The substances represented are plagioclase (10), augite (7), magnetite, serpentine 

 (6) and interstitial matter. 



The felspars of this rock belong to two periods of consolidation. They consist 

 of (a) porphyritic crystals and crystalline granular aggregates, and (b) crystals of the 

 groundinass giving lath-shaped sections. 



The porphyritic felspars, one of which is represented in the upper part of the 

 figure, are allied to anorthite in composition and optical characters (see Q.J.G.S., 1884. 

 p. 234). They contain solid inclusions and often show a zonal structure. 



The felspars of the groundinass usually show binary or multiple twinning. They 

 are probably labradorite. 



The augite is nearly colourless in thin sections. It occurs in grains and granular 

 aggregates. The opaque iron-ores are almost, if not entirely, limited to the interstitial 

 matter. They occur as rods and granules. 



As a rule serpentine is absent from this rock. One dark patch, probably 

 representing olivine, occurs in the figure somewhat to the right of the centre. 



The interstitial matter is crowded with rods and granules of opaque iron-ore, 

 ill-defined brownish granular matter and microlites of felspar. A considerable portion 

 of it is seen, in very thin sections, to be isotropic, but as it does not appear absolutely 

 structureless it must be referred to the micro-felsite of IXOSKNHUSCH. 



The rock forms a dyke in Carboniferous strata. The date of intrusion cannot, 

 however, be determined with precision. 



