Yol.  49.]  PROF.  J.  W.  JUDD  ON  COMPOSITE  DYKES  IN'  ARRAN. 
545 
Fig.  1. — Diagrammatic  Plan  of  the  Cir  Mhor  Dyke . 
x=  Granite. 
a  =  Augite-andesite. 
b  =  Quartz-felsite. 
c  =  Pitckstone-porpky  r y. 
|  Quartz-pantellerite. 
By  tke  kindness  of  my  colleague,  Prof.  Tkorpe,  F.R.S.,  I  kave 
keen  able  to  obtain  analyses  of  the  two  varieties  of  rock  of  this 
dyke ;  these  were  made  under  his  superintendence  in  the  Chemical 
Laboratory  of  the  Boyal  College  of  Science,  South  Kensington. 
In  order  that  the  identity  of  the  vitreous  and  stony  varieties  of 
the  acid  rock  might  be  made  perfectly  clear,  analyses  were  obtained 
both  of  the  quartz-felsite  and  of  the  4  pitehstone-porphjwy.’ 
The  specific  gravities  of  these  rocks  have  been  determined  for  me 
with  very  great  care  by  my  assistants,  Mr.  G.  Cullis  and  Mr.  T. 
Barron. 
I. 
Ferrous  oxide 
Soda 
Potash 
by  ignition 
Augite-andesite 
II.  Quartz-felsite 
III.  ‘  PlTCHSTONE- 
Trap-porphyry  ’). 
(‘  Clay  stone-porphyry'). 
PORPHYRY.’ 
; tside  of  Cir  Mhor 
Centre  of  Cir  Mhor 
Centre  of  Cir  Mhor 
Dyke. 
Dyke. 
Dyke. 
...  55-79 
75-31 
72-37 
...  15-97 
13-62 
11-64 
...  12-50 
2-31 
1*42 
1-08 
...  7  06 
6-97 
1-30 
...  2-22 
0-20 
0-52 
...  2-21 
3-02 
4-15 
...  1-86 
4-07 
3*98 
...  0-45 
trace 
— 
|  2-43 
1-48 
4-86 
100-49 
100-98 
101*32 
.  2*70-2-71 
2*52-2*53 
2*36-2-37 
2  0  2 
