Vol. 60.] IGNEOUS ROCKS OF THE BRISTOL DISTRICT. 153 



ealcite, with sometimes in addition a brown, possibly chloritic, 

 mineral occurring in collections of irregular spherulites. Mr. Teall'a 

 description of a lava from Spring Cove, quoted by Sir Archibald Geikie 

 & Mr. Strahan, 1 is as follows : — 



'The lava from Spring Cove, Weston-super-Mare [E. 3212 (23)], is a fine- 

 grained, chocolate-coloured rock, composed of pseudomorphs after olivine, and 

 probably augite, in a groundmass showing microlitic structure. The pheno- 

 crysts are represented by pseudomorphs in carbonate. The microlitic felspars 

 of the groundmass are colourless, but they no longer show their characteristic 

 optical properties. The groundmass is deeply stained with ferric oxide. 

 Although all the minerals have been destroyed, the structure has been perfectly 

 preserved, and there can be no doubt whatever that the original rock was an 

 olivine- basalt.' 



Most of our sections from Spring Cove did not show the carbonate- 

 pseudomorphs above referred to, which are so clearly seen in the 

 Goblin-Combe rocks, but they were met with in one section. 



(3) Summary of the Characters of the Lavas. 



The rocks vary a good deal in coarseness, the coarsest-grained 

 being those from Uphill and from near Cadbury Camp, of the con- 

 temporaneous character of which there is no direct evidence. The 

 Goblin-Combe rocks come next in degree of coarseness, while the 

 finest-grained are those from Spring Cove and Milton Hill. 



All the rocks are clearly basaltic in character, and consist 

 essentially of plagioclase-needies, laths, or phenocrysts, with pyro- 

 xene and iron-ore, and, as a rule, olivine. The pyroxene is 

 generally undoubtedly augite, but sometimes, as in the Uphill 

 rocks, the unaltered mineral may have been enstatite. The freshest 

 augite is seen in the rock from the eastern end of Goblin Combe, 

 where it occurs both in small grains and large plates. Plates of 

 fairly-fresh augite are seen in the Cadbury-Camp and one of the 

 Milton-Hill rocks; and serpentinized pyroxene, probably augite, is 

 abundant in the rock from the western Goblin-Combe exposure. 



Olivine is never preserved in an unaltered state in these rocks, 

 though in most cases the original form of the crystals is very well 

 seen (Goblin Combe — eastern end, Milton Hill, AVoodspring). 

 Sometimes the olivine is completely converted into bright green 

 serpentine (Goblin Combe — eastern end), sometimes a very large 

 amount of ferric oxide is associated with the serpentine (near 

 Cadbury Camp), sometimes the olivine is replaced by pseudomorphs 

 in carbonate, probably ealcite, with which dense masses of ferric 

 oxide are associated (Milton Hill), sometimes it is apparently 

 simply replaced by pseudomorphs in carbonate (Goblin Combe — 

 western exposure, and some specimens of the Spring-Cove rock). 



The occurrence of long needles of peroxidized magnetite is a 

 characteristic feature of the Goblin-Combe rocks. 



The variolitic character of much of the Spriug-Cove lava is an 

 interesting feature. 



1 ' Summary of Progress of the Geological Survey for 1898 ' p. 106. 



