ON THE CONSTITUTION OF THE GRANITES OF DONEGAL. 638 
III. A crystalline greasy trap rock, forming @ dyke which penetrates the 
lower arenaceous Carboniferous limestone, and expands into a mass 
on the top of the hill, close to the town of Donegal. It contains 
black hornblende and a green feldspar. 
IV. The analyses of the feldspar and hornblende of this rock have already 
been given. The precise locality of the specimen analyzed is 
unknown; but it resembles very closely the syenites of the Black 
Gap and Ballykillowen. It forms a link between the granites and 
the syenites. It contains— 
(a). Quartz. 
(6). Oligoclase of a pinkish-yellow colour; large crystals, brilliant 
cleavage. 
(c). Hornblende, dark glossy blackish green, passing into black 
mica : 
(d). Occasional crystals of sphene. 
Determination of the Mineralogical Composition of the Donegal Granites*. 
The determination of the mineralogical constitution of a granite is a pro- 
blem whose solution has been frequently attempted; and at first sight it does 
not seem to present features of extraordinary difficulty. It may be stated as 
follows :— 
Given the chemical composition of a rock and of its supposed constituent 
minerals, it is required to find the proportions in which these minerals are 
associated in it. 
In the case which is now under our consideration, we assume that the four 
minerals, quartz, orthoclase, oligoclase, and black mica, all of which have 
been observed in the granite, are its constituents, and we take their chemical 
composition as ascertained by the analysis of specimens obtained exclusively 
within the county Donegal. 
The principle of this investigation has already been published by Professor 
Haughton, in the paper already referred to (Quart. Journ. Geol. Soc. Lond. 
vol. xviii. p. 403), and its complete discussion will be laid by him before the 
Geological Society of London at an early date. It may, however, be of 
interest to lay before the Association the results at which he has arrived. 
By the conditions of the question, we obtain four equations to determine 
the same number of unknown quantities, as will be seen by a reference to the 
paper. From the coefficients of the several quantities we obtain, by actual 
multiplication, the following ten constants:— 
Bx 69077. Bp =—188220. y = 196708. 
a’ =—113443. p= 211768. y=") -7Ol6r: 
Boss 1702: p"=— 24442. y"=—163140. 
K=34131692. 
Once these constants have been determined, nothing but simple multipli- 
cation and division is required, in order to ascertain at once with absolute 
certainty the percentage of each of the four minerals, whose analysis has been 
given, in the granite, ¢. g.:— 
Percentage of orthoclase=10000 { se a ‘ 
The results of the application of these constants to each of the seventeen 
granites whose analysis is contained in Table Y. is given in Table VII. 
* This portion of the Report is solely due to Professor Haughton. 
