CALCISPH@RA. 109 
point in question. This resolves itself into a case of careful 
microscopic observation. It is within the knowledge of all who 
have carefully studied the Carboniferous Microzoa, that minute 
organisms have their several parts more or less encrusted with a 
calcareous covering, which conceals many of the minor details. 
Hence it would not be surprising if the aperture in the case 
of a Radiolarian skeleton were obscured. Here, as regards 
the microscopic appearance, I would prefer to quote from 
ProrF. SoLtas, who in a letter says “‘On putting a section of the 
limestone under the microscope with a power of 500 diameters, 
I see plainly a number of dark coloured rings, each surrounding 
a clear crystalline area, and enclosed by the more opaque 
matrix outside. (The crystalline material affects polarized 
light.) The dark rings are not continuous, as a section across 
a continuous spherical shell would be, but interrupted, and in 
some places net-like in character ; from which I infer that they 
are sections across a spherical network. In many cases a 
number of dark lines are prolonged from the interior of the 
dark rings radiately; these appear to be spines. It is un- 
necessary to say that all these appearances are consistent with 
a Radiolarian structure, and not with anything else.” The 
observations of Pror. SoLtas and. Pror. W. C. WILLIAMSON 
are certainly conflicting, but capable of being verified one 
way or the other. 
(c) The possibility of the displacement of Silica by Calcite, 
and the conversion of silicious organisms into calcareous, is, I 
am aware, somewhat of an innovation upon the teaching of the 
past. Hence, Pror. WILLIAMSON quotes an authority to the 
effect—‘‘ That I know no agency by which silicious structures 
could be converted into calcareous.” The modus operand? may 
even now not be too well understood, but the fact discovered 
by Sottas and ZiTret, that the Silica in the structures of 
various organisms has been wholly replaced by Calcite, has 
now a well-established reputation, and does not rest upon an 
isolated case, and is not limited to a particular geological era, 
It has been observed in cases ranging from Palzozoic to 
Tertiary times. The fact is that Silica, in the form in which 
it exists in organic structure, is not the intractible element it 
was formerly supposed to be. ‘Take the case of the Diatoms of 
the London Clay. The deposit of these silicious organisms 
extends for miles as a thick band at the base of the clay, and the 
whole of the Silica originally present has been removed, and 
replaced by ferric sulphide. It is not merely a coating, but a 
solid replacement (Journ. R. Micr. Soc., Ser. II,, Vol. I., p. 6.) 
The number of silicious organisms, available for reference in 
illustration of this change of substance, is necessarily limited ; 
since, with the exception of the Sponges and Diatoms, 
there are no silicious bodies of geological importance. The 
Diatoms are limited to the later geological periods: there 
remain then only the Sponges to which we can refer. 
