134 Proceedings of the Boyal Society 
reasons for believing that the invisible side of the earth’s crust is 
very irregular in its structure ; and that, as pressure promotes 
solidification, the great internal mountains must constantly increase 
in depression in consequence of the deposition on their peaks of 
solid matter of low density, and consisting either wholly or largely 
of silicic acid. I ascribed earthquakes to the occasional instability 
of such masses of new rock, as their size and buoyancy causes them 
to Jbreak loose from their fastening, and an ascending stony 
avalanche is driven against the weaker parts of the earth’s crust. 
But, on taking into consideration the great affinity of silicic acid 
for bases at a high temperature, volcanic phenomena may be traced 
to the collision of these silicious avalanches against such sedi- 
mentary rocks as contain carbonic and many other acids. Car- 
bonate of lime, for instance, would not be decomposed by heat under 
the pressure it feels at great depths; but if a stratum of limestone 
were struck by a mass of incandescent quartz, or of highly silicified 
rocks, the resulting fragmentary mass would swell with the evolu- 
tion of carbonic acid, and give rise to the various peculiarities 
observed in the eruptions and the upheaval of volcanic mountains. 
5. On the Placentation of the Sloths. By Professor Turner. 
After referring to the paucity of information on the placental 
characters of the sloths, and to the various inferences which had 
been drawn by anatomists from Carus’s figure of the placenta of 
Bradypus tridactylus, some holding that it was cotyledonary and 
non-deciduate, others that it might have intermingled with it 
maternal deciduous substance, the author proceeded to describe 
his dissection of the perfectly fresh gravid uterus of a specimen 
of a two -toed sloth. This specimen, which was presented to him 
by Dr David Eidpath, only possessed six cervical vertebrae, and was 
referred to the Cholcepus Hoffmanni of Peters. 
The author had succeeded in obtaining excellent injections 
both of the foetal and maternal systems of blood-vessels. The 
placenta consisted of about thirty discoid lobes, aggregated to- 
gether, and occupied about fths of the surface of the ovum. These 
lobes could be peeled off the placental area of the uterus, and 
carried away with them a layer of deciduous serotina, the curling 
