PLATE XXXII. 



FIG. 1. 

 MICA-DIORITE. 



PEN VOOSE, THE LIZARD, CORNWALL. 



Magnified 25 diameters. Drawn with polarizer only ; short axis right and left 



The minerals represented are reddish-brown mica (9), felspar (10) and 

 hornblende (8). 



There is a rude kind of parallelism in the arrangement of the mica flakes. 

 The colourless portions of the figure split up under crossed nicols into an aggregate 

 of large and small grains. These are sometimes striated ; more frequently 

 unstriated. Sections cut approximately at right angles to an optic axis give a 

 biaxial figure, thus proving that the mineral in question is not quartz. Horn- 

 blende occurs sparingly and is allotriomorphic. In other portions of the slide 

 there are plagioclase crystals with zonal structure ; the interior of these crystals 

 is more altered than the exterior. 



The rock in question occurs in association with gabbro and granite. 



Fia. 2. 

 MICA-TRAP. 



SWINDALE BECK, NEAR APPLE13Y. 



Magnified 50 diameters. Drawn with polarizer only ; short axis right and left 



The only conspicuous minerals represented are a dark mica (9) and 

 pyrites (19). 



The ground-mass is an aggregate of quartz, decomposed felspar and calcite 

 dust. 



This figure illustrates the characteristic form of the mica of the mica-traps. 

 It is biaxial with a small optic axial angle. Sections at right angles to the basal 

 plane usually show a lamellar structure due to twinning. 



Pyrite occurs in irregular patches. The felspathic constituent which doubt 

 less entered largely into the composition of the rock has been so altered that it 

 is impossible to determine its character. Quartz occurs in small grains and is 

 probably a secondary product. 



The rock forms a dyke in Lower Palaeozoic strata. 



