1903-4.] Mr J. G. Goodcbild on Intrusive Rocks. 
215 
Fig. 26 has been drawn up so as to afford a conspectus of the proportions in 
which the Essential Minerals of the Eruptive Rocks occur in any one of 
the sections into which the whole lithological series can be divided. For 
example, taking the second band, the proportions in which the plagioclase 
felspars occur relatively to the ferro-magnesian silicates in any one of either 
the sub-basic or the basic eruptive rocks, can be estimated by comparing 
the distance above the thick curved line traversing the middle with that 
below, measured at any point along a line perpendicular to the base of 
the diagram. The same method can be employed in the case of any 
other of the subdivisions of the series. 
The principle of arrangement followed is based, primarily, upon the 
percentage of silica present — the rocks containing highest percentage 
being represented at the top left-hand, and those with the lowest at 
the bottom right ; and, secondarily, wdth reference to the nature of the 
dominant alkali, or alkaline earth, which characterises each of the 
compounds. 
The classes of rocks formed of these components may be grouped under 
three primary categories, to each of which one subdivision of the diagram 
is devoted. At the top are represented the Mineral Combinations arising 
from the action of a Potash Magma upon other rocks in which the 
dominant alkali is Soda. The middle of the diagram includes those 
which are here regarded as due to the action of a Soda-Lime Magma 
upon sedimentary rocks. The lowest subdivision is intended to represent 
the products of consolidation of a Ferro-magnesian Magma. Further 
subdivisions, which are sometimes convenient for use, are made in 
accordance with the dominant substance, and are as follows : rocks 
characterised by minerals containing Potash, Potash-Soda, Soda, Soda- 
Lime, Lime-Soda, Lime, Lime-Magnesia, Magnesia. 
The graphical method here employed can be used also to illustrate the 
proportions of each of the mineral constituents present in the Aplites (or 
more acid segregations of each group), as well as those of the Pegmatites 
and Gneisses whose composition allies them to that of their massive 
prototypes. 
