792 : REPORT—1896. 
the consolidation of the chalk took place concurrently with deposition ; for example, 
bands of rolled nodules of chalk occur at varions horizons, and the same is probably 
the case with the Globigerina ooze of the existing oceans, for the ‘Challenger’ 
dredged nodules of hardened ooze from a depth of 1,700 fathoms. 
The author concludes that the Upper Chalk was probably deposited in a depth 
of at least 1,500 fathoms, a conclusion which Dr. Hume and Mr. Jukes Browne 
appear to have reached by entirely different methods. 
9. The Highwood Mountains of Montana and Magmatic Differentiation. 
A Criticism. By H. J. Jounston-Lavis, W.D., #.G8., &e. 
The author brings forward a new interpretation of the facts described by 
Messrs. W. I. Weed and L. V. Pirrsson (‘ Bull. Geol. Soc.,’ America, vol. vi. 
pp. 389-422, pts. 24-26) in their account of the remarkably interesting volcanic 
region of the Highwood mountains, with reference, more especially, to Square 
Butte. ; 
This mountain they show to be a dismantled laccolite intrusion into Cretaceous 
sandstones. The peripheral part of this intrusion is composed of a dark basic rock, 
that they call shonkinite, containing about 47 per cent. of silica, poor in alumina 
and alkalies, but rich in iron, lime, and magnesia. The core is composed of a white 
syenite containing about 57 per cent. of silica, is rich in alumina and alkalies, but 
poor in iron and alkaline earths. The authors conclude, therefore, that this is a 
case of magmatic differentiation in which the bases have concentrated to the sides 
by a process of diffusion or liquation. 
The author suggests that what really took place at Square Butte was as 
follows: In the first stage a conduit containing a paste sensibly approaching the 
syenite in composition was injected into the Jurassic and other basic sedimentary 
rocks subjacent to the Cretaceous sandstone, which forms a more superficial part of the 
original country. Here the upper intratelluric portion of the intrusion underwent 
basification by interosmotic action with the conduit walls. In the second stage this, 
now shonkinite, paste or magma was pushed on and formed a blister or laccolite 
in the sandstone smaller than the complete one of Square Butte. This, having 
undergone partial lapidification and becoming highly viscous, was in turn pushed 
up and aside by the intrusion of the syenite. This latter paste had probably re- 
mained a shorter time in the conduit, the walls of which had already been in part 
exhausted in osmotic interchange or diffusion by the earlier batch of paste that 
had remained in contact with them, and had been so basified to the composition 
of shonkinite. In consequence of this the second batch, which formed the syenite 
mass, was less or entirely unchanged in composition. 
The peculiar plate-like structure of the peripheral portion, which is erroneously 
attributed by the authors to cracking, set up parallel to the isotherms of cooling, 
is, in fact, evidence of shearing planes or fluxion structure in a viscous mass the 
homogeneity of which was not perfect at the time of its being stretched over the 
uprising boss of syenite. The phenomenon is met with in domes of all viscid 
magmas, and is beautifully shown in the island of Basiluzzo; the writer suggests 
that the cleavage of gneiss, forming mantles to granite intrusions, may have also 
so arisen. 
The partial fusion together of the shonkinite and syenite shows that the former 
was yet very hot, as indicated by the plasticity that must have existed to allow of 
the formation of the concentric shear-planes referred to. Ilad the shonkinite not 
been to some extent plastic it would have been more fractured, and fragments of it 
would have become enveloped in the syenite. 
The shonkinite, however, was in that state of which the author first showed 
the important bearing in volcanic rocks, and which may conveniently be called 
viscous inertia, in which a viscous body responds instantaneously to a shock as if 
it were a solid. The shonkinite, although plastic, was at such a critical point that 
when it was suddenly stretched out over the back of the new syenite intrusion it 
