64 GEOGNOSY. © «ss | Booed 
limestone and gypsum. These strata, sometimes full of remains of 
fresh-water shells and plants, are interlaminated with sulphur, the — 
very shells being not infrequently replaced by this mineral, Here 
the presence of the sulphur may be traced to the reduction of the 
calcium sulphate to the state of sulphide through the action of the 
decomposing organic matter, and the subsequent production and 
decomposition of sulphuretted hydrogen, with consequent liberation of 
sulphur! The sulphur deposits of Sicily furnish an excellent illus- 
tration of the alternate deposit of sulphur and limestone. They 
consist mainly of a marly limestone, through which the sulphur is 
partly disseminated and partly interstratified in thin lamine and 
thicker layers, some of which are occasionally 28 feet deep. Below 
these deposits lie older Tertiary gypseous formations, the decompo- 
sition of which has probably produced the deposits of sulphur in the 
overlying more recent lake-basins.* 
The weathering of sulphur is exemplified on a considerable scale 
at these Sicilian deposits. The sulphur, in presence of limestone, 
oxygen, and moisture, becomes sulphuric acid, which combining 
with the limestone forms gypsum, a curious return to what was 
probably the original substance from the decomposition of which the 
sulphur was derived. Hence the site of the outcrop of the sulphur 
beds is marked at the surface by a white earthy rock, or borseale, 
which is regarded by the miners in Sicily to be a sure indication of 
sulphur underneath, as the gossan of Cornwall is indicative of under- 
lying metalliferous veins.® 
Tron.—This most important of all the metals has hitherto been 
found only sparingly in the native state. It occurs in grains and 
blocks which have fallen from planetary space as meteorites. Nor- 
denskiold describes fifteen blocks of iron on the island of Disco, 
Greenland, the weight of the two largest being 21,000 and 8,000 
kilogrammes (11°8 and 7:9 tons) respectively. Numerous smaller 
pieces have been picked up in most parts of the world; fine grains 
or dust of similar iron haye been observed in hailstones and in snow 
of the Alps, Sweden and Siberia, and by Mr. Murray of the 
Challenger on the ocean floor at remote distances from land. There 
can be no doubt that a small but constant supply of native iron 
is falling upon the earth’s surface from outside the terrestrial 
atmosphere.* ‘This iron is alloyed with nickel, and contains small 
quantities of cobalt, copper and other ingredients. Dr. Andrews, 
however, showed in 1852 that native iron in minute spicules or 
granules exists in some basalts and other volcanic rocks,> and 
Mr. J. Y. Buchanan has recently detected it in appreciable quan- 
tity in the gabbro of the West of Scotland. It occurs also in 
Braun, Bull. Soc. Gel. I’rance, 1st ser. xii. p74; 
Memorie del Rt, Comitato Geologico d Italia, i. (1871). 
Journ. Soe, Arts, 1873, p. 170. 
See Ehrenberg, Mrorieps Notizen, Feb. 1846 ; Nordenskivld, Comptes-rendus Acad. 
Sei. Ixxvii. p. 463, xxviii. p. 236. 'Tissandier, op. cit. 1xxviii. p. 821, Ixxx. p, 58, Ixxxi. 
p. 576, See Ixxy. (1872) p. 683. Yung, Bull. Soc. Voudoise, Sei, Nat. (1876), xiv. p. 4938. 
* Brit. Assoc. Iep. 1852. 
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