348 THE AMERICAN NATURALIST. | [Von. XXXVI. 
cooling of the mass.". The original body of magma was leucocratic, 
so that the alumina. and alkalies with. silica enough for the forma- 
tion of feldspar and. feldspathoids, crystallized first, and hence in the 
periphery of the mass. - 
Densities of Liquid and Solid Rock Magmas. — In view of the 
fact that a recently proposed theory of volcanic action accounts for 
the eruption of lava as the consequence of the expansion of liquid 
magma in its passage to the solid state, a paper by Doelter,! in which 
this author discusses the densities of liquid and solid magmas, 
becomes of great interest. By means of a series of experiments, in 
the course of which fragments of known density were allowed to float 
or sink in molten magmas, Doelter obtained a series of results which 
are embodied in the following table, the figures indicating specific 
gravities : 




NATURAL 
SUBSTANCE. Morrzw. GLASSY. CRYSTALLINE. 
Melanite 325 3.55-3.6 3-55-3-6 3-65-3-7 
ugite . 33 2.92 2.92-2.95 3.2.—2.25 
burgite 2.83 2.55—2.57 2.55-2.57 2.75-2.78 
Lava (Ætna) 2.83 2.58-2.74 2.71-2.75 2.81-2.83 
Lava (Vesuvius) 2.84 2.68-2.74 2.69-2.75 2.77-2.81 
Nephelinite . 2.74 2.70-2.75 2.686 2.72-2.75 
Leucitite. 2.83 2.60-2.68 2.68-2.72 2.75-2.79 





The Laccolite of Shefford Mountain.?— Shefford Mountain is the 
easternmost of the series of nine hills of igneous material that rise 
above the St. Lawrence valley in the neighborhood of Montreal. It 
is thought by Dresser to be an old laccolite in Lower Cambrian and 
Trenton sediments. Its material consists of essexite, nordmarkite, 
and pulaskite, the first two of which possess almost the typical 
character of these rocks. Associated with these are dark-colored 
dykes of a camptonitic. type, and others of a theralitic type, and light- 
colored ones of trachyte and bostonite. The latter are the younger. 
All the rocks are thought to be differentiated products of a single 
magma. The primary magma, according to this view, had nearly the 
composition of pulaskite. - Excluding the dykes the first differentiate 
was tlie basic essexite, the second was the acid nordmarkite, and the 
third the intermiediate pulaskite. The analyses of the essexite (I), 
a Wekes Jahrb. of Univ., etc., Bd. ii (1901), p. 141. 
? Amer. Geologist, vol. xxvii (Oct., 1901), p. 205. 
