10 T. STERRY HUNT ON THE GENETIC 
chlorhydric acid, and remains suspended in the solution, but is easily separated by wash- 
ing from the pale grayish-green silicate. This equals about three-hundredths of the 
weight of the limestone. When ignited in the air it assumes a bright fawn color, and 
under a lens contrasts strongly with the colorless grains of quartz with which it is mixed. 
Its chemical characters were like those of hamelite, and analyzed in the same manner it 
gave, after deducting 21.0 per cent. of insoluble sand, the following composition: Silica, 
35.72, alumina 22,26, ferrous oxyd 21.42, magnesia 6.98, potash 1.49, soda 0.67, water 
11.46 = 100.00.” This gives for protoxyds, alumina, silica, and water very nearly the 
atomic ratios 3:4: 7:4; but we are not sure of its homogeneous character. A silicate 
very like this in aspect and mode of occurrence has been found in a band of fossiliferous 
limestone near the base of the coal-measures in southern Ohio, but has not yet been 
chemically examined. 
§ 8. In connection with these minerals should be noticed a greenish fibrous asbesti- 
form silicate, elsewhere described by the writer, which occurs in veins traversing the 
anthracite and the carbonaceous shales of the coal-measures at Portsmouth, Rhode Island, 
either without admixture or mingled with pyrites, or penetrating white quartz, and also 
coating the fragments of the crumbling disintegrated anthracite. It is a hydrous silicate 
of alumina, ferrous oxyd, magnesia, and alkalies, more basic than those above described, 
yielding the atomic ratios of 4 : 4: 6: 3, and, though differing in structure, is near to pro- 
chlorite or voigtite in composition. ? 
$ 9. We have elsewhere explained how solutions which would otherwise have 
yielded zeolitic minerals or epidote may, by exchanging their lime aud alkalies for mag- 
nesia and ferrous oxyd, have given rise to aluminous double silicates like those just des- 
cribed. In like manner, non-aluminous solutions which might have yielded pectolite, 
apophyllite, or related silicates, by exchange with magnesian or ferrous solutions, may 
give origin to silicates like serpentine, sepiolite, and probably to glauconite. The mag- 
nesian silicates just named occur, as is well known, in aqueous deposits, by themselves or 
mingled with carbonate of lime, in strata of palæozoic or even of cenozoic age, while ser- 
pentine fills the Eozoon of more ancient times. 
§ 10. The probable relations between the protoxyd-silicates and glauconite are worthy 
of notice. By the latter name is designated a soft greenish amorphous mineral sometimes 
found in the cavities in basic amygdaloidal rocks, but more abundantly in sandstones and 
marls, among which it often forms beds, with but little admixture, and is commonly called 
“green-sand.” It is well known that glauconite is met with filling the shells of foraminifera 
and other marine organisms, from early geological times, and even occurs in the same 
manuer in recent foraminifera in various seas. The mode of its occurrence in these cases 
is similar to that of the aluminous double silicates in organic forms from limestones, as 
described above. The composition of glauconite is very variable; and, while essentially 
a hydrous silicate of potash and iron-oxyd, it may contain of alumina from one or two up to 
twelve hundredths or more, and of magnesia from traces up to six hundredths. Indeed, 
a so-called green-sand from the calcaire grossier, according to Berthier, is rather a highly 
ferrous serpentine, containing, silica 40.0, ferrous oxyd 24.07, magnesia 16.6, lime 8.8, 
alumina 1.7, water 12.6 = 98.9. 

! Amer. Jour. Science, 1871, ii. 57. * Trans. Roy. Soe. Can., Vol. iii. Sec. iii. p. 70. 
* Beudant, Traité de Minéralogie, ii. 178. See also Report Geol. Survey of Canada, 1866, p. 231. 
