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in the early part of the nineteenth century. As evidence of the 
porosity of the earth’s crust, Mr. Hora drew attention to the 
celebrated Florentine experiment of the sixteenth century, in 
which the water contained in a thick hollow spherical gold shell 
was forced through the solid by decreasing the volume of the ° 
hollow interior by changing the sphere’s shape. Moreover, the 
lecturer remarked, still more wonderful were the experiments of 
D’Aubigny, who 40 years ago showed that water by capillary 
attraction only could be made to penetrate almost anything. 
Thus it could be easily understood that a constant supply of 
water would be passing gradually through the ocean bottom into 
the solid crust below, where it would be converted into super- 
heated steam at an enormous pressure. 
The theory not only accounted satisfactorily for the remark- 
able coincidence of regions of volcanic activity with those of 
seismic activity, but it gave a very beautiful explanation of 
mountain formation. 
The lecturer drew attention here to the modern view of the 
earth’s interior, viz., that it was probably a solid core and a thin 
solid crust, but that between the crust and the core might be a 
thin layer of molten lava, ready to flow out on to the earth 
wherever there was a deep fissure or volcanic break which 
permitted its egress. Elie de Beaumont, about 1830, sought to 
explain the formation of mountains by the contraction of the 
earth’s crust due to secular cooling—the “dried apple skin” 
theory as it was sometimes termed—a theory which, though it 
had held the field for many years and was simple to understand 
and apparently supported by many mechanical experiments to 
illustrate it, yet seemed likely to be abandoned, after the destruc- 
tive criticism levelled against it by the Rev. O. K. Fisher (in his 
book ‘‘ Physics of the Earth’s Crust”), Professor Suess, Sir 
Archibald Geikie, and others, in favour of the idea that mountain 
chains have arisen from subvolcanic explosions of steam, and 
‘that during seismic activity a tangential thrust towards the land 
from the sea causes the lava below the crust to be forced up near 
the seashore, and thus a chain of mountains results. Slides were 
shown to illustrate this idea, and also how table lands or elevated 
plateaux could be explained in the same manner. Mr. Hora 
stated that as a corollary all mountain ranges must be of volcanic 
origin, and to this statement the Alps and Himalayas seemed 
exceptions, but it was only right to state that these are quite 
recent upheavals and that the original volcanic axes may be 
concealed and buried beneath the thick masses of the nummulitic 
limestone which make up these mountains. 
The rival theory, originally stated about 1831 by Bous- 
singault, and now supported by a large band of seismologists, 
such as Professor Milne in England and Major Dutton in the 
United States, is known as the ‘‘ Tectonic” theory and supposes 
