210 ALEXANDER N. WINCHELL 
rocks belonging to a given series along this co-ordinate are placed 
within a single rectangle on successive sheets. For example, gab- 
bro, essexite, and theralite form a series of this kind, the whole 
series to be observed simultaneously by looking through trans- 
parent paper for the second and third members. Along this 
co-ordinate igneous rocks are classified as normal or alkali-calcic 
(conveniently abbreviated to alkalcic), alkaline, and peralkaline. 
Subsiliceous alkalcic rocks are characterized mineralogically by 
the absence of feldspars and feldspathoids and the presence of bio- 
tite, olivine, ferromagnesian (or mafic"), or calcareous-ferromagnesian 
(or calmafic), amphibole or pyroxene (or two or more of these). 
They are distinguished chemically by very low content of 
alkalies and alumina and high tenor of magnesia, lime, and iron. 
Mediosiliceous alkalcic rocks are characterized mineralogically by 
the presence of soda-lime feldspar nearly or wholly to the exclusion 
of alkali feldspar, and by the presence of biotite, olivine, mafic, or 
calmafic amphibole or pyroxene; they contain more soda than 
potassa (K,O) and more than enough alumina to saturate the 
alkalies. Persiliceous alkalcic rocks are marked by the presence 
of soda-lime feldspar with dominant alkali feldspar, and with mus- 
covite, biotite, or mafic or calmafic amphibole or pyroxene; chem- 
ically they are distinguished by the presence of both alkalies and 
lime with more than enough alumina to saturate the alkalies. 
In general, alkalcic rocks are characterized by the presence of 
soda-lime feldspar (or no feldspar) with mica, olivine, mafic or 
calmafic amphibole or pyroxene, and the absence of feldspathoids 
(or lenads), soda-amphiboles, soda-pyroxenes, and lithia micas; 
they contain both alkalies and lime with more than enough alumina 
to saturate the alkalies. 
Subsiliceous alkaline rocks are distinguished mineralogically by 
the absence of feldspars and lenads, and the presence of sodic and 
titaniferous amphiboles and pyroxenes; chemically they contain 
little alumina, and relatively large amounts of alkalies, titanic acid, 
1 The new terms felsic and mafic are here used in accordance with the proposal 
of Cross, Iddings, Pirsson, and Washington, Jour. Geol. XX (1912), p. 560, as short 
general terms, the first applying to feldspathic minerals and quartz, or rocks rich in 
such minerals, the second applying to all ferromagnesian minerals or rocks rich in 
such minerals. 
