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One is calculated on the hypothesis that the mineral contains 
no other alkali than potash, the other on the hypothesis that it 
contains soda only. 
“The fact is, that we rarely, if ever, find a felspar whose 
composition coincides altogether with either of these calculated 
analyses, both the alkalies being, we may say, always present, 
but in almost every case (with a few exceptions specified in 
the standard works on mineralogy) one of them is found to 
preponderate greatly over the other, and accordingly the name 
orthose, or potash felspar, has been generally assigned to those 
specimens whose chemical character is defined by a great ex- 
cess of potash, while the name albite, or soda felspar, has been 
given to those in which soda exists in great excess. There 
are other distinctions founded on crystallographic form, which 
at present I purposely abstain from dwelling upon, my present 
object being to lay before the Academy the results of the 
chemical examinations I have made of this important consti- 
tuent of our granite rocks. 
‘The ordinary felspar occurring in granite is orthose or 
potash felspar, and in those cases in which this mineral is re- 
placed or accompanied by albite, the granite is designated 
albitic. In Ireland we have a very interesting case of this 
description in the granite of the Mourne mountains. And 
accordingly, we frequently find it cited both by English and 
foreign writers on geology as a typical locality. The granite 
district which stretches in a south-westerly direction from 
Williamstown, in the county of Dublin, through the county 
of Wicklow, to Poull Mounty, in the neighbourhood of New 
Ross, has been, up to a very late period, supposed to furnish 
that variety of felspar only which is called orthose or potash- 
felspar. Any statement, therefore, to the contrary, if made on 
competent authority, must naturally attract attention; and 
any amount of pains or trouble may be said to be well expended 
if we can arrive at the determination of a question of such high 
interest to the geological inquirer. 
