788 REPORT—1896. 
to the subdivision, for they all occur in the underlying Middle White Limestone. 
The number in the list is less than in the others, on account of the Upper Grey 
Limestone having been considerably denuded in the Vale of Clwyd. 
Nearly the whole of the common and very common fossils occur in each of the 
three lists, for there are few that are not found in all the areas. 
_ Of the numerous common aud very common species found in the Carboniferous 
Limestone of North Wales, it is impossible to find any that are restricted to 
horizons of Jess importance than the subdivisions into which the formation is 
naturally divided. An examination of the first appearance and continuity of the 
species seems to indicate that they were introduced from some pre-existing area, 
and that the upper beds cf the formation are more recent than in Derbyshire and 
Yorkshire, where the thickness of the Limestone is very much greater. 
The sudden appearance of species in restricted areas, like those found in the 
Upper Grey Limestone at Axton, in Flintshire, where 20 species occur, and at 
Graig-fawr, in the Middle White Limestone, where 6 species occur, not found 
elsewhere in North Wales; and the early appearance of 3 species in beds of black 
limestone and shale at the base of the Middle White Limestone at the Great 
Orme’s Head seem to indicate migration from some other area. The latter species 
are Orthis Michelina, Spirifera humerosa, and S. rotundata. Spirifera humerosa had 
only been previously found at Llangollen and in Flintshire, while S. rotundate 
is rare in North Wales ; but none of the 3 species had been previously found at a 
lower horizon than the Upper Grey Limestone. Productus giganteus first appears 
in the Lower Brown Limestone, and very large specimens occur within 50 feet 
from the base at Moel Hiraddug, a few miles from Rhyl. The species occur all 
through the Carboniferous Limestone, and thousands may be seen in the Upper 
Grey Limestone. 
In this paper the range of the species found is confined to North Wales, but 
when the subdivisions of the Carboniferous Limestone in other parts of the 
eountry are worked out, and the species from each tabulated, it will be interesting 
to compare the result with that obtained in North Wales. 
5. On the Source of Lava. By J. LoGaN Losrey, L.G.S., Professor of 
Astronomy and Physiography, City of London College. 
The object. of this paper, which was illustrated by diagrams, was to show that 
small columns of lava cannot pass through thirty miles of earth crust, and that 
therefore the source of lava cannot be at that distance from the surface, as is so 
often assumed 
The reasons adduced were : 
First, that from the pressure of overlying rocks there can be no fissures 
giving a passage to lava below ten miles from the surface, since this pressure, 
much greater than the crushing weight of rocks, would cause lateral extension 
where possible. 
Secondly, if even a way were open, lava rising from a source thirty miles 
deep, would by contact with cooler rock masses lose its fluidity at twenty miles 
from the surface. The temperature of lava at i's source cannot be very much 
greater than that of the contiguous solid rocks, and lava would lose heat continuously 
and increasingly as it ascended the voleanic conduit. The temperature at twenty 
miles below the surface is much under rock-fusion temperature, and the lava- 
eolumns giving small or even moderate emissions are so insignificant in volume 
that they would there be so cooled as to solidify. Estimates of the volume of 
lJava-columns were given in illustration; and it was further shown that a column 
of lava 300 feet in diameter and thirty miles high would require a dynamic force 
ef 820,800,000 tons to sustain it even without ejection. 
The author’s conclusion is that lava is not derived from! a central source, but 
' Brit, Assoc. Report, 1888, p. 670. 
