298 Proceedings of the Royal Society of Edinburgh. [Sess. 
much so in the Fintry phonolite that nepheline to the extent of 7 ‘4 per 
cent, is developed in place of quartz. All the rocks are highly felspathic, 
and, of the various felspar molecules, albite is the most abundant, ortho- 
clase is developed in subordinate quantity, but anorthite in very insig- 
nificant amount. This corresponds to the observed abundance of albite 
and soda-orthoclase in the rocks described. The development of quartz, 
felspars, and felspathoids in igneous rocks is largely controlled by the ratio 
between silica and alkalies in the magma ; and this relation is expressed 
in the American Quantitative Classification by the ratio between quartz 
or felspathoids, and felspars (see Table II). The rocks in this series fall 
Table II. 
1 
1. 
2. 
4. 
6. 
7. 
8. 
9. 
10. 
, 
Quartz .... 
12-5 
17-0 
9-7 
•4 
3-8 
15*0 
28-8 
Orthoclase 
10-6 
18-4 
19-4 
30 6 
31-7 
36-7 
30-0 
23-9 
Albite .... 
56T 
49-2 
46-6 
45-6 
55-0 
43-5 
43-0 
28-3 
Anorthite 
3-6 
2*2 
1-9 
1-4 
5*3 
5*8 
Nepheline 
7-4 
... 
Corundum 
5T 
1-9 
... s 
Acmite .... 
2-3 
4-2 
... 
Diopside 
3-0 
4T 
4-7 
•5 
2-2 
1-4 
Hypersthene . 
6-2 
•6 
4-7 
3-8 
4-7 ! 
Wollastonite . 
•7 
•9 
Magnetite 
2'8 
1-2 
2-8 
6-0 
1-9 
•2 
37 
Ilmenite 
3-5 
1-0 
4T 
•5 
IT 
•6 
•3 
Haematite 
6*2 
5-6 
4-6 
Apatite .... 
•3 
1-0 
•3 
•3 
Titanite .... 
•4 
Class, Sal/Fem ratio 
5-2 
6 1 
5-8 
6-8 
11*8 
12-6 
8*1 
8-3 
Order, Quartz or Lenad 
•09 
*005 
•04 
•49 
T7 
•25 
•14 
•20 
Felspar 
! 
Fang, K 2 0 + Na 2 0/Ca0 . 
9*7 
CO 
15-5 
24*0 
32-4 
7-8 
QO 
4-6 
Subrang, K 2 0/Na 2 0 
T7 
•35 
•39 
•49 
•54 
•80 
•66 
•80 | 
in this respect within the fourth and fifth orders of the system. The 
relative abundance of alkali-felspars (orthoclase and albite), with respect 
to lime-felspar (anorthite), is expressed by the ratio of K 2 0 + Na 2 0 to 
CaO used in making felspars and felspathoids, and is given by the figures 
for this ratio in Table II. The rocks are all peralkalic with the exception 
of the Lennoxtown felsite, the ratio of alkalies to lime being over 7 to 1. 
Hence, with the above exception, all the rocks fall into Rang 1 of the 
American Quantitative Classification. The ratio between albite and 
orthoclase (expressed by the ratio between salic Na 2 0 and K 2 0) is used 
as the basis of the subrangs of the system. As shown by the ratios in 
Table II the rocks range from dosodic (the albite-bostonites) to sodi- 
