O'Eeilly — On the Occurrence of Idocrase in Co. Monaghan. 447 
In Zirkel's llth edition of Naumann^ s Mineralogy ^ 1881, p. 528, 
the constituents of Yesuvian or Idocrase are given as ranging in the 
best analyses, as follows : — 
37 
to 
39 
13 
)) 
16 
Ferric oxide, . . 
. . 4 
J) 
9 
33 
5) 
37 
0 
J) 
1 
The figures resulting from Mr. Spencer's analysis are practically 
within these limits. Zirkel adds that, in the cherry -blood red variety 
from Johnsberg, Silesia, 3*23 of MnO was determined by Yon Lasaulx. 
Dana gives a series of analyses for the mineral, and mentions the 
variety, " XantTiite'^ a yellowish -brown Yesuvian from near Amity, 
Y., which contains 2-8 MnO, while a manganous variety from 
St. Marcel, Piedmont (where ores of manganese occur), has a sulphur- 
to honey-yellow colour, which fairly corresponds with that of the 
Monaghan specimen. This may, therefore, be regarded as a Xanthite 
variety of Idocrase. The presence of water, from 0 to 2*73 per cent., 
is noted in the series of analyses given by Dana ; and its presence is 
usually attributed to partial decomposition of the Idocrase mineral. 
This decomposition is quite apparent in places in the Monaghan 
specimen. 
As Idocrase is not of common occurrence in Ireland, it may be 
interesting to note the localities in which it has been already observed. 
(For the earlier notices of the mineral I am indebted to Dr. Hyland, 
of the Geological Survey.) 
1812. Stephens-Fitton, ''Notes on the Mineralogy of part of the 
Yicinity of Dublin " (London, 1812), p. 45. Yesuvian (Idocrase, Hauy), 
Kilranelagh, Co. Wicklow, seemingly in a loose boulder, the author 
observing, that " appearances render it probable that they were not 
far from the natural place." 
1826. Giesecke, "Account of a Mineralogical Excursion to the 
Centre of Donegal," p. 23. Barony of Boylagh[and Bannogh, bacil- 
lary Idocrase of yellowish-brown and greenish-yellow colour from 
Lough Anure, in the Bosses, N.-W. Donegal. (This is a vein in 
granite.) 
1858. Greg and Letsom, in their '' Manual of Mineralogy," p. 101, 
mention as Irish localities, Derryloaghan, Barnes' Gap, near Kilma- 
crenan, near Lettermacherward ; Bunbeg, Gweedore (in crytalline dolo- 
mite). 
1861. Bristow, in his " Glossary of Mineralogy," mentions as Irish 
