Sullivan — On Augite and Hornhlende. 
37 
Surveyor for the southern division of the county of Dublin. Mr. Gray 
was passing through Glencullen on Wednesday, the 9th of this month, 
when he noticed that great quantities of snow were melting on the 
sides of the mountains, and that the streamlets were filled to overflow- 
ing. It immediately occurred to him to look at the water parting, to 
which I had called the attention of the Academy, and which it appears 
he had previously noticed ; and he there saw the single feeding stream 
from Glendoo rushing down in great volume, and the two bifurcating 
streams parting from it and from each other in the most palpable manner. 
The abundance of water was such as to partly flood the adjoining road. 
Mr. Gray is decidedly of my opinion, that the beds of these streamlets 
are not due to artificial cuttings, but are the result of the physical con- 
formation of the glen. 
X. JN'OTE ON THE HoENBLENDE AND AuGITE GeOUPS OF MlNEEALS, 
By William K. Sullivan, Ph. D., Secretary of the Academy. 
[Read April 25, 1870.] 
JText to the feldspar group, the hornblende and augite groups are the 
most important, from the point of view of lithology and petrography. 
The minerals included in these groups belong to the same crystalline 
system, but to difi'erent crystalline series; in chemical composition they 
approach so closely that the typical varieties of each group may be 
represented by. the same general formula; and, lastly, the minerals of 
each group belong chemically to two classes — 1. silicates of the dyad 
metals, magnesium, calcium, iron (ferrosum), and manganese, or simple 
augites and hornblendes ; 2. the aluminous augites and hornblendes. 
The nature of the relationship of these groups has not yet been clearly 
established, while the nature of the aluminous silicate or rather sili- 
cates, and the way the latter are present in the mineral, are still 
obscure. Having occupied myself with the study of these groups for 
some time, though in consequence of other occupations in such a 
desultory way, that I have not been able to bring the inquiry to a 
satisfactory conclusion as yet, I am induced, chiefly in consequence 
of a recent memoir of Herr Tschermak, in which he has incidentally 
stated his views upon the two points just stated, to briefly lay before 
the Academy the conclusions to which my study of the groups lead 
me upon those same points. 
Dr. Tschermak, who has been so successful in unravelling the 
difiiculties of the feldspar group of minerals, states in his prize essay 
on the porphyritic neozoic rocks of Austria,^' that typical hornblende, 
as represented by tremolite, has the formula Ca" Mg/ Si4 O12; and 
typical augite, represented by diopside, the formula Ca"Mg"Si2 06. 
He considers that the aluminous hornblendes contain, in addition to the 
* Die Porphyrgestcine Oesterreichs aus der mitleren geologischen Epoche 
Wien, 1870. 
