104 
Note on Silicious Incrustations from the 
convenience, the vessel of ammoniacal sulphate of copper patronized 
by Count Castracane. 
At first, I wished to construct, without any modification, 
Mr. Collins’s accessory plate, which for the optical effect may be 
placed in the microscope more advantageously than my apparatus ; 
but to keep the chief plate of the least possible thickness is so 
powerful a consideration, that I decided on adopting the plan which 
I have just described. 
In order to substitute one glass for another when examining 
an object, I have no need, as with Mr. Collins’s arrangement, to 
change my position ; and I can observe, without losing sight of the 
object, what is the most favourable colour for the perception of 
certain details peculiar to it. 
However, I am not yet sufficiently acquainted with the advan- 
tages to be derived from those glasses. They will, perhaps, become 
useful auxiliaries to micrographists, by enabling them to make more 
frequent use of solar light, without experiencing the least fatigue. 
IV . — Note on Silicious Incrustations from the Geysers of the 
Yellowstone Paver, Wyoming Territory. 
By Prof. Arthur Mead Edwards, U.S.A. 
The specimens are of two characters. First, we have a mass of 
white stony material, arborescent in appearance and looking as if 
it were some species of vegetation, as a moss, converted into a 
mineral. The others are rounded masses, smooth on the exterior, 
white, and evidently consisting of the same substance as the arbo- 
rescent specimen. These were given to’ me by Mr. P. T. Austen, 
of the School of Mines, who has made a chemical analysis of them 
with the following result : — 
Si0 2 87-23 
Fe 2 0 3 and Al„ 0 3 1 • 13 
CaO .. .. ' '55 
MgO -46 
PCD 1-48 
99-94 
For my own part, being interested in the subject of the mode 
of formation of concretions of the character of these rounded masses, 
I determined to sectionize one of them and examine it by means of 
the microscope, to ascertain, if possible, what kind of nucleus it had. 
This matter is one of importance on account of its bearing upon the 
