﻿•part 1] IGNEOUS ROCKS OF THE BRISTOL DISTRICT. 



35 



four of which were examined, the bulk of the rock is seen to he 

 composed of felspar-laths, which tend to he larger than in the 

 Globlin- Combe rocks and in those of Milton Hill and Spring Cove. 

 The felspars, though much weathered, are seen to extinguish 

 practically straight, and their refractive index is higher than that 

 of Canada balsam. Hence it is probable that they are oligoclase. 

 Needles of peroxidized magnetite, similar to those occurring in 

 some of the Goblin- Combe rocks, are sometimes present. Pseudo- 

 morphs after olivine in an iron-stained carbonate are plentiful. 

 Partly owing to the extensive weathering, and partly to the ferru- 

 ginous staining, no unaltered augite was seen ; but, in one section, 

 small serpentinized augite-grains filling the interstices between the 

 felspars are easily recognizable. 



(c) Limeridge Wood, Tickenham. 



Hand-specimens from this locality are fairly fresh-looking dark- 

 brown rocks, generally crowded with calcite- or chlorite-filled 

 amygdules, though sometimes non -vesicular. Eight slices were 

 examined. In all, the main part of the rock consists of felspar- 

 latbs generally about 05 mm. long, and, as a rule, too much 

 weathered for the extinction-angle to be ascertainable. When, 

 however, the rock is a little less weathered, the extinction-angle is 

 found to be practically straight. Associated with the felspar is 

 interstitial . augite much weathered and obscured by iron-oxide. 

 In nearly every section regularly-idiomorphic pseudomorphs after 

 olivine are abundant : in some cases they are in a carbonate, in 

 others in serpentine. 



The specific gravity of three specimens ranged from 263 to 2'68. 

 The silica-percentage of one rock (76) is 4392, the soda is 044, 

 and the potash 4*03. The high percentage of potash and the 

 straight extinction suggest that the bulk of the felspar is ortho- 

 clase. Amygdules formed of calcite, or partly of chlorite, partly 

 of finely-spherulitic chalcedony, are common. 



The rock is closely related to that of Uphill. 



(d) Spring Cove and Milton Hill. 



Spring Cove. — The Spring-Cove lava is a very fine-grained 

 red rock in which practically nothing of the nature of the 

 constituent minerals can be ascertained without microscoj^ical 

 examination. The amygdules and red variolitic patches, which 

 (as already mentioned) 1 are a characteristic feature, are very 

 irregular in their distribution. Each variolitic patch is a group of 

 imperfect varioles. The macroscopic characters of the rock, and, 

 in particular, the occurrence of imperfect pillows 3 and of varioles, 

 suggest that this rock may be related to the spilites. The specific 

 gravity is also low for basalts. Specimens taken from about the 



1 Q. J. Gr. S. vol. lx (1904) p. 152. 



2 See ' Summary of Progress for 1893 ' Mem. Geol. Surv. 1899, p. 105. 



D 2 



