1862.] HAXJGHTON GRANITES OF IRELAND. 419 



If, now, Q., Or., 01., and M. denote the per-centages of quartz, ortho- 

 clase, oligoclase, and mica in this granite, we have the following 

 equations from the preceding oxygen-ratios : — 



375100=5192 0.+3281 Or. + 3111 OZ. + 1878ilf. (1) 



74600= 891 Or. + 1141 OZ. + 1628 M. (2) 



25000= 355 Or,+ 361 Ol.-{- 394 if. (3) 



To which we may add the following : — 



100 = Q. + Or. + 0Z. + if. (4) 



the reason of which is evident. 



From these four equations, the four unknown per-centages are to 

 be determined. 



Eliminating if. from (2) and (3), we find 



1130800=22689 Or. + 13817 01 {a.) 



Also, eliminating Q, from (1) and (4), we obtain 



144100 =1911 Or, + 20810?. +3314 if ...... (6.) 



Again, eliminating M. between (6) and (3), we obtain the following 

 equation in Or. and OL : — 



2607600=42357 Or. +37650 01 (c,) 



Solving equations (a) and (c) for Or. and 01., we obtain 



OZ.=41-88. 

 Introducing these values of Or. and 01. into equation (2), we find 



if=3-16. 



And, finally, from (4) we obtain 



^=30-63. 



The mineralogical composition, therefore, of the granite of Doo- 

 charry Bridge is — 



Doocharry Bridge Granite, 



Quartz 30-63 



Orthoclase 24-33 



Oligoclase 41-88 



Black mica 3-16 



100-00 



The preceding calculation leaves little to be desired in point of 

 accuracy, although it is open to the objection that it is somewhat 

 laborious. I believe it to be superior in accuracy to the method of 

 measurement used by Delesse. This distinguished geologist has 



VOL. XVIIl. PART I. 2 F 



