106 Physical Theories of the Earth — Eeade. 
described by Prof. Fontaine from leaf or cone impressions, 
and it would, of course, be of great interest if these could be 
correlated, but until they are found organically connected this 
is manifestly impossible. This state of affairs is, however, no 
reason why both leaves and wood should not be named and 
described, for we shall now have two sets of criteria which will 
enable us to make out the life history of the genus and also 
furnish valuable remarks for stratigraphic determination. 
The species, although having evident affinities with several 
described forms, are all regarded as new, and have been named 
as follows: Cupressinoxylon pulchellmn , G. Wardi , C . McGeei 
and C. Columbianum. 
Araucarioxylon. The genus to which I have referred the 
other species, has also been regarded as a comprehensive one 
although recent investigations of Grand, Eury, Felix, Morgen- 
roth, and others make it probable that it must be again divided. 
It represents in a fossil state the wood of the Araucarian pines 
of Australia and the Pacific islands. It is characterized by the 
hexagonal areolatioos on the wood-»cells, by the absence of 
resin ducts and faint line of demarcation between the annual 
rings of growth. 
The species I have called Araucarioxylon virginianum. It 
was obtained from Taylorsville, Va. 
CONCLUSION. 
The conclusions reached in this paper are briefly as follows: 
The fossil wood of the Potomac formation is all coniferous. 
It exists under two different conditions, viz: as lignite, which 
owing to the great pressure to which it has been subjected, is 
much metamorphosed and distorted and is incapable of specific 
determination; and as silicified wood. The latter material, very 
perfectly preserved, belongs to two genera, Cupressinoxylon 
with four species and Araucarioxylon with a single species. 
U. S. National Museum, Washington , A C. 
PHYSICIAL THEORIES OF THE EARTH IN RELATION TO 
MOUNTAIN FORMATION. 
By T. Mellard Reade, C.E., F.G.S., F.R.I.B.A. 
In an article entitled ‘’On some investigations regarding the 
condition of the interior of the Earth,” in the June and July 
