GENESIS OF THE ALKALINE ROCKS 109 
of trachytic and syenitic magmas generally.'. The solution of 
small amounts of Mesozoic and Paleozoic non-calcareous sediments 
(or of their connate waters) in basaltic magma may be primarily 
responsible for the quite subordinate, alkali-rich lavas of Queens- 
land. The traces of calcareous material in these sediments might 
co-operate in the underground reactions, but carbonates were not 
in chief control.’ 
SPECIAL CHEMICAL CONSIDERATIONS 
“Saturation” in igneous rocks.—Shand expresses a suggestive 
idea in distinguishing ‘‘saturated,” “‘undersaturated,” and ‘‘over- 
saturated” igneous rocks.3 Saturated rocks are those that contain 
only minerals which are capable of forming in the presence of free 
silica. Any rock containing free quartz or tridymite of magmatic 
origin is said to be oversaturated. Undersaturated rocks are com- 
posed, wholly or in part, of minerals unsaturated with silica. 
Shand’s list of unsaturated minerals includes leucite, nephelite, 
sodalite, nosean, analcite, cancrinite, hauyne, melanite, melilite, 
magnesian olivine, corundum, and perovskite—all characteristic 
components in alkaline rocks.4 
One of the causes for undersaturation Shand finds in assimila- 
tion. He writes (p. 511): 
When the invaded rock is a carbonate or other non-silicate rock, or con- 
tains much lime, magnesia, or iron in the form of oxide or carbonate, then the 
advantage as regards absorbing power lies with the saturated and oversaturated 
magmas, which can yield first their excess of silica, and secondly a further 
quantity of silica due to the reduction of sodium, potassium, calcium, and 
magnesium molecules from the saturated to the unsaturated state. In this 
way a saturated or oversaturated magma may become undersaturated. 
™R. A. Daly, Igneous Rocks and Their Origin (New York, 1914), pp. 303, 410. 
2 The remark of Richards (p. 190), that ‘Dr. Jensen has also advocated the assimi- 
lation of carbonate rocks by the parent magma with the resultant production of 
alkaline material,” is hardly a correct rendering of Jensen’s hypothesis. Jensen 
assumes the precipitation of ‘‘alkaline salts” to the floor of the primitive ocean, their 
burial, and their later fusion and fluxing with the silicates of the overlying rocks. 
The present writer has not “‘elaborated this view,” as Richards states. 
3. J. Shand, Geol. Mag., X (1913), 508. 
4See J. Morozewicz, Tschermaks Min. und Petr. Mitt., XVIII (1808), 224. 
A. Holmes (loc. cit., p. 71) points out that the basalts associated with the alkaline 
lavas of East Africa are also undersaturated with silica. 
