SCIENCE-GOSSIP. 



221 



basalt is a particularly dark green — almost black ; 

 the vesicular beds are a dark brownish green, 

 while the tuffs, etc., are a dirty sage green. All 

 are exceedingly weathered and decomposed; in 

 m 5t cases the result is that their colours are 

 darkened, but in some instances I noticed the 

 vesicular bands of a reddish hue, probably 

 due to the decomposition cf the iron. Their 

 structure and weight are also different, the 

 ordinary basalt being heavy, dense, exceedingly 

 tough, and hard. The vesicular formation is 

 naturally comparatively lighter, and of a more 

 open texture. Some of the vesicles were filled 

 with calcite and have a great resemblance to the 

 •■ Toadstone" of Derbyshire. The tuffs and ag- 

 glomerates themselves of course vary in texture 

 and weight, some of the finer kinds being so far 

 decomposed as to crumble in the hands. 



An examination of a thin section of what appears 

 to be the normal basalt under the microscope 

 shows the rock to consist of a ground base 

 composed of small lath-shaped crystals of felspar, 

 patches of brownish augite, with flakes and minute 

 cubes of magnetite, throughout which are scattered 

 crystals of idiomorphic plagioclase, augite, 

 olivine, and larger patches of magnetite. The 

 ;rys:s of felspar exhibit twinning on the 

 albite and pericline system, and contain inclusions 

 of fluid cavities as negative crystals. The ground 

 mass in many cases shows a fluxional arrangement 

 of the little lath-shaped crystals around the large 

 crystals. Some of the olivine exhibits signs of 

 decomposition, small specks and streaks of serpen- 

 tine appearing. Pseudomorphs of calcite also 

 occur in this section. 



On examining a thin slice of rock taken from 

 one of the vesicular beds mentioned, I found a 

 ground base resembling that of the basalt pre- 

 viously described. The phenocrysts, however, 

 which are felspar and augite are very much 

 posed. This rock shows itself to be very 

 m the first-mentioned. Being taken 

 Tar the surface of a ridge of vesicular lava, 

 tea to contain numerous small spheroidal and 

 ■■%, many of which are filled in with 

 calcite These cavities are generally lined ith 

 ■■:. and the calcite filling up 

 the remainder of the space shows cleavage and 

 double refraction very strongly. This rock has 

 petition to some extent, the 

 I maw being very dark and the minerals 

 generally ditncult of identificat: 



most interesting mir.' r.ng in this 



I which i-. 



•nalcime ' ai small ( 



or elli: 



'I h>; 

 are all of ;i fibrODI radiating char.v ler, 



vesicles, but occasionally they occur completely 

 isolated and of perfect form. In the calcite they 

 are seen in some instances to entirely fill up the 

 cavities, and then the spherulitic structure can 

 only be observed with the aid of the polarizer. 

 Viewed with the polarizer these spherulites show 

 very strong pleochroism, polarizing vividly in 

 many colours. The circular ones exhibit a faint 

 bluish cross, while the elliptical ones show two of 

 these. 



A microscopic examination of the fragmentary 

 rocks show them to be in nearly all cases very 

 much decomposed. Chlorite is generally present 

 in fairly large quantities, lining all the cavities in 

 fibrous fringes, and sometimes filling up the whole 

 of the interstitial space between the fragmentary 

 matter. The ground mass is generally very dark, 

 and occasionally opaque, the lath-shaped crystals 

 being again present, but very small. The large 

 felspar crystals which appear are very much de- 

 composed, as is the olivine which is present. 



After walking for some distance over this 

 volcanic sheet the scenery changes, and the wild, 

 rugged basaltic rocks give place once more to the 

 gently sloping beds of limestone. At the head of 

 Poolvash Bay there is a marble quarry where an 

 exceedingly hard dark-coloured marble occurs in 

 thin beds and accompanied by similar beds of 

 shale. The rock is not unlike Derbyshire black 

 marble, and is said to take almost as high a polish. 

 It is from a quarry in this vicinity that Bishop 

 Wilson obtained the black marble which he 

 presented to St. Paul's Cathedral, and from which 

 were constructed the flights of steps at its western 

 and southern doors. The quarry is not a very 

 large one, and does not appear to be worked at the 

 present time. 



All along the coast from Poolvash Quarry to 

 Port St. Mary the rock formations are very 

 peculiar and interesting. The sea at this point is 

 gradually washing away the land, and for a long 

 distance there is an exposed section of about 

 fifteen feet in depth. At one point this consists of 

 boulder clay, of which there is such a great 

 amount all over the island. This boulder clay 

 differs somewhat from the more ordinary drift of 

 the island, being composed of a very stiff red clay 

 containing large pebbles and boulders. These 

 stones are not nearly so numerous as in the ilrifi 1 

 noticed in other parts of the island. 



Near the hamlet of Strandhall there is a peculiar 



i gritstone about i u oi five feet thick, ami 



of a very deep purple colour. The beds of this 



Imgly wavy, looking like gigantic 



ripple marks, and much larger than any 1 have 



observed. The gril i't a remarkably coat te 



one, and gel er down, being in fat I a 



fine coi i erate al the bottom, 1 he rod does 



i er y fai on eithet sidi , and Is 



