Vol. 60.] 



IGNEOUS ROCKS OF FONTTESFORD HILL. 



465 



sharp outlines, set in a matrix of very fine glass}- dust, containing 

 in places much secondary calcite. The lapilli of palagonitized glass, 

 often covered with minute brown pigment-spots, like spots on a 

 leopard's skin, show no reaction with crossed nicols. The round 

 vesicles have a clear transparent border of a doubly-refracting 

 zeolite, and a faintly-polarizing substance in the centre, while many 

 elongated vesicles have a yellow border of palagonite, and enclose a 

 colourless zeolite in the centre. Broken crystals of felspar, measur- 

 ing up to 0*02 inch, occur, as also occasional angular grains of 

 quartz. 



Fig. 3. — Sketch-map shoving Pontesford rods faulted against 

 Cambrian shales, at the top of the road hading to Pontesbury 



[2011 



Palagronite-Tuffs. Intrusive Dolerite. 



Tuffs. 



■^ r 



r 





20i(a x ,b,c,d.) v 



,> \-X\N^\\ 2C1 



<2X>I 



in 



nKnN^^.I 



20I'- V f 



Fault 



[Scale: 1 inch = about 20 feet.] 



Xo. 201 b, though occurring quite close to 201 a, is very different 

 in colour and texture. It is made up of lapilli of vellowish-green 

 vesicular palagonite and crystal-fragments. A fragment of brown 

 glass (0*0025 inch) contains minute needles of felspar and larger 

 laths of the same mineral showing distinct lamellar t winning 

 together with small green patches that may be decomposed pyroxene. 

 Lapilli, measuring up to 0*1 inch across, vesicular, slaggy, and 

 twisted, are common, with some secondary calcite in the matrix. 



Xo. 201 d is harder, and paler in colour with pink and green 

 flecks, and contains lapilli of greenish andesite-glass with minute 

 black vesicles. Microscopically, it shows good felspar-crystals, 



