18 T. STERRY HUNT ON THE GENETIC 
olite, on the other. Moreover, if, as is probable, there are conditions under which pyr- 
oxene may be separated in à similar manner from the feldspathic element, we should 
have a farther differentiation, giving rise to heavier and highly pyroxenic portions on the 
one hand, and to lighter and more feldspathic portions on the other. 
$ 27. The careful student of crystalline rocks will have noticed in ntaure many 
examples of variations in different portions of eruptive masses, which find a ready explana- 
tion in a process of partial solidification and eliquation, as suggested by Durocher and 
illustrated by the experiments of Fouqué and Michel Lévy. This is well displayed in 
certain rocks intruded among the Ordovician strata of the St. Lawrence valley, near Mon- 
treal, and forming the hills known as Rougemont, Montarville, and Mount Royal. These, 
as I have long since described them, are essentially doleritic, but present very great dif- 
ferences in the proportions of their mineralogical elements in contiguous parts. Thus in 
some portions of these masses we have a pyroxene and labradorite rock, in which these 
two elements are pretty equally distributed; while in other portions the rock is almost 
wholly a black, coarsely crystalline pyroxene, with but an insignificant proportion of the 
feldspathic element. Elsewhere the arrangement of these two species gives rise to strati- 
form structure. 
§ 28. As described by me in 1863,’ for Mount Royal, “ mixtures of augite and feld- 
spar are met with, constituting a granitoid dolerite, in parts of which the feldspar pre- 
dominates, giving rise to a light grayish rock. Portions of this character are sometimes 
found limited on either side by bands of nearly pure black pyroxenite, giving at first 
sight the aspect of stratification. The bands of these two varieties are found curiously 
contorted, and ...seem to have resulted from movements in a heterogeneous pasty mass, 
which have effected a partial blending of an augitic magma with one more feldspathic in 
nature.” In the doleritic mass of Montarville, the alternation of a coarse-grained variety 
of dolerite, porphyritic from the presence of large crystals of pyroxene, with a finer-grained 
and whiter variety, is noticed, the two “being arranged in bands whose varying thick- 
ness and curving lines suggest the notion that they have been produced by the flow and 
the partial commingling of two fluid masses.” Of this stratiform structure, it was then 
said, it seems to be due to “ the arrangement of crystals during the movement of the half- 
liquid crystalline mass, but it may in some instances arise from the subsequent formation 
of crystals, arranged in parallel planes.” 
§ 29. The feldspars mentioned, as shown in the published analyses by the writer, 
are near in composition to labradorite. The composite rocks described also contain, be- 
sides pyroxene, more or less magnetite and menaccanite, with chrysolite. This last spe- 
cies is for the most part distributed ‘sparsely, through these rocks, but occasionally, like 

1 Geology of Canada, 1863, pp. 665, 667, and Amer. Jour. igre 1864, xxxvili. 175-178. 
* Farther illustrations of this were given by the author in a communication to the Boston Society of Natural 
History, January 7,1874: “ Among these was a specimen shown from Groton, Connecticut, in which a large angular 
fragment of strongly banded micaceous gneiss is enclosed in a fine-grained eruptive granite, the mica plates in 
which are so arranged as to show a beautiful and even stratification in contact with the broken edges of the gneiss, 
but at right angles to the strata of the latter. Another example is afforded by the eruptive diabase from the meso- 
zoic sandstone of Lambertville, New Jersey, which is conspicuously marked by light and dark bands, due to the 
alternate predominance of one or the other of the constituent minerals; and still another is a fine-grained dark 
micaceous dolerite dike from the Trenton limestone at Montreal, in which the abundant laminæ of mica (probably 
biotite) are arranged parallel to the walls of the dike.” Chem. and Geol. Essays, p. 186. 
