﻿part 1] 



IGNEOUS BOOKS OE THE BEISTOL DISTRICT. 



37 



The next point where the lava is exposed is at the cross-roads at 

 the top of Milton Hill. The highest bed seen here, when the 

 exposure was in a better condition for observation than it is now, 

 was a rock which in a hand-specimen was not obviously amygda- 

 loidal, although it shows many ill-defined patches of a green 

 alteration-product. Numerous pseudomorphs in carbonate after 

 olivine occur with iron-oxide aggregated along their margins and 

 cracks. Underlying this is a coarsely amygdaloidal rock with 

 ■caicite-filled vesicles sometimes an inch and a half long. There are 

 other vesicles filled wholly or in part with a chloritic mineral, while 

 •other amygdules consist of calcite enclosing a nucleus of quartz. 

 It is difficult to ascertain whether certain spots are amygdules or 

 imperfect varioles. The bulk of the rock consists of felspar-laths 

 of somewhat variable size, which extinguish straight or nearly so, 

 and have a lower refractive index than the Canada balsam in which 

 they are mounted. These characters indicate orthoclase, the 

 presence of which is further suggested by the high percentage of 

 potash — namely, 5 -83, found by Mr. Radley (Mr. Sturgess found 

 5*34). Much iron-oxide is present. The phenocrysts include 

 large (often idiomorphic) olivines, represented by pseudomorphs in 

 ■carbonate associated with much iron-oxide, and a few felspars 

 which have very low extinction-angles. Some of the large felspars 

 have isotropic patches which Dr. H. H. Thomas suggests may be 

 due to analcitization. 1 There are also isotropic patches in the 

 ground-mass which are probably analcite. The specific gravity of 

 the rock (94) is 2*57 to 2 - 58. A close resemblance exists to the 

 Spring-Cove rock, particularly in the high percentage of potash. 

 A trace of lithia was also observed. 



The easternmost exposure on Milton Hill is that found by 

 Dr. A. Strahan near Florence Cottage, 700 yards east-north-east of 

 the cross-roads. This exposure occurs in the bed of a small pond 

 about 150 yards west of the Cottage, and in wet weather nothing 

 •can be seen. The rock is very highly vesicular, and so much 

 weathered that no sections were cut and the specific gravity was 

 not ascertained. As has been already pointed out, it closely 

 resembles the rock (102) from the little pond on Milton Hill, the 

 rock in each case probably coming from quite the bottom of the 

 flow. 



(e) Woodspring. 



The Woodspring trap in a hand-specimen is a much- weathered 

 dark-greenish rock with numerous amygdules, which in the. bulk 

 of the rock are small, rounded, and filled with green material. 

 In places very large calcite-amygdules occur. In thin sections 

 the main part of the rock is seen to consist of small felspar-laths 

 in a greatly weathered state; sometimes, however, it is possible to 

 satisfy one's self that the extinction is practically straight. Much 



■} Analcitization in albitized basic felspars is described by E. B. Bailey & 

 W. Grabham from Arthur's Seat, Edinburgh, Geol. Mag. dec. 5, vol. vi 

 (1909) p. 254. 



