130 
Proceedings of the Royal Irish Academy. 
and magnesia equal to 1 "35 of carbonate — the remainder being carbonate 
of lime. The insoluble matter separated by dilute acid, after washing 
by decantation from a small amount of fine flocculent matter, consists, 
apart fi'om an admixture of quartz grains, entirely of casts and moulded 
forms of a peculiar silicate, which Dr. Dawson has observed in decal- 
cified specimens filling the pores of crinoidal stems, and which, when 
separated by an acid, resembles closely under the microscope the coral- 
loidal forms of arragonite known as flos ferri, the surfaces being some- 
what rugose and glistening with crystalline faces. This silicate is 
Bub-translucent and of a pale green colour, but immediately becomes of 
a light reddish brown when heated to redness in the air, and gives off 
water when heated in a tube, without, however, changing its form. 
It is partially decomposed by strong hydrochloric acid, yielding a 
considerable amount of protosalt of iron. Strong hot sulphuric acid 
readily and completely decomposes it, showing it to be a silicate of 
alumina and ferrous oxide, with some magnesia and alkalies, but with 
no trace of lime. The separated silica, which remains after the action 
of the acid, is readily dissolved by a dilute solution of soda, leaving 
behind nothing but angular and partially rounded grains of sand, 
chiefly of colourless vitreous quartz. An analysis efii'ected in the 
way just described on 1*187 grammes gave the following results, 
which give, by calculation, the centesimal composition of the mi- 
neral : — 
Silica, . 
. -3290 . 
. 38-93 
Alumina, 
. -2440 . 
. 28-88 
Protoxyd of iron, 
. -1593 . 
. 18-86 i 
Magnesia, . 
. -0360 . 
4-25 { 
Potash, . 
. -0140 . 
1-69 ( 
Soda, 
. -0042 . 
•48; 
Water, . 
. -0584 . 
6-91 
Insoluble, quartz 
. -3420 . 
M869 
100-00 
6-29 
6-14 
A previous analysis of a portion of the mixture by fusion with 
carbonate of soda gave, by calculation, 18-80 p. c. of protoxide of iron, 
and amounts of alumina and combined silica closely agreeing with 
those just given. 
''The oxygen ratios, as above calculated, are nearly as 3 : 2 : 1 : 1. 
This mineral approaches in composition to the jollyte of Yon Kobell, 
from which it diff'ers in containing a portion of alkalies, and only one 
half as much water. In these respects it agrees nearly with the sili- 
cate found by Eobert Hoffman, at Easpenau, in Bohemia, where it oc- 
curs in thin layers alternating with picrosmine, and surrounding masses 
of Eozoon in the Laurentian limestones of that region;^' the Eozoon itself 
being there injected with a hydrous silicate which may be described as 
intermediate between giauconite and chlorite in composition. The 
* "Jour, fiir Prakt. Chemie," Bd. 106 (Erster Jahrgang, 1869), p. 356. 
