284 IGNEOUS ROCKS 0¥ THE TOETWORTH INLIER. [Aug. I9OI, 



IV. Conclusions. 



We believe that we have confirmed or established the following 

 points with regard to the igneous rocks of the Tortworth inlier : — 



1. That well-marked tuffs occur, and that the trap-rocks are, both 



by inference and by internal evidence, lava-flows of the 

 nature of pyroxene-andesites or basalts. 



2. That they occur on two horizons which follow the north- 



eastern and northern boundaries of the Bristol Coalfield. 



3. That the lower one is overlain by beds of Upper Llandovery 



age, and the upper one by beds of Lower Wenlock age. 



EXPLANATION OF PLATES X & XI. 



Plate X. 



Geological sketch-map of the igneous rocks and associated sedimentaries of 

 the Tortworth inlier, on the scale of 3 miles to the inch, 



Plate XL 



Microscope-sections of andesites and tuffs. 



Fig. 1. Calcareous ash from Middle Mill, Tortworth district. Xl8. This 

 shows a rounded lava-fragment, embedded in a matrix chiefly com- 

 posed of quartz-gi-ains, but showing much carbonate of lime. 



2. Pyroxene-andesite from north of Daniel's Wood, Tprtworth district. 



X 33. This shows needles and laths of plagioclase, varying con- 

 siderably in size ; also part of a large, much-altered felspar-pbenocryst, 

 boi'dered by a zone of fresh secondary material. Also a crystal of 

 fresh augite, and by it a basiite-pseudomorph after enstatite, en- 

 closing the ends of some of 1;he felspar-laths in an ophitic fashion. 



3. Pyroxene-andesite or basalt with quartz-grains, from Daniel's Wood, 



Tortworth district, x 16. This shows two gi-ains of quartz corroded 

 by the groundmass, and surrounded by a thick border chiefly com- 

 posed of pyroxene. The groundmass shows fresh needles and laths 

 of plagioclase, the latter varying considerably in size, and numerous 

 small grains of pyroxene. 



4. Calcareous ash from Cullimore's Quarry, by the railway north of Char- 



field Station. Xl4. This shows numerous angular fragments of 

 trap of several types, with quartz-grains and crystals of altered 

 felspar embedded in a calcareous matrix, 

 [No. 1 is from the lower band, and the other three are from the upper baud.] 



Discussion. 



Prof. W. W. Watts pointed out the difficulty of establishing the 

 contemporaneity of volcanic activity in any district, and particularly 

 in association with Silurian rocks. The specimens of tuffs exhibited 

 by the Authors appeared, however, to be genuine contemporaneous 

 tuffs, and the Authors were to be congratulated on having established 

 a new chapter of volcanic activity in Britain. 



The Chaieman (Mr. H. W. Moncktoi^) and Gen. C. A. McMahon 

 also spoke, and Mr. Reynolds briefly replied. 



