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Proceedings of the Royal Society of Edinburgh. [Sess. 
matter, negative with crossed nicols. Contained within it — or, perhaps 
more strictly speaking, cemented together by it — are minute particles, and 
these by interference give it finely granular texture. In some instances 
the clear matter itself appears to be in the form of minute grains of 
irregular shape. Foraminiferal cells have become infilled with this material 
as well as clear silica, and remain in the acid residue as casts. After treat- 
ment of the acid residue with heated caustic potash, it became evident that 
the silica contained in this rock had advanced towards crystallisation to a 
greater degree than could be suspected by the examination of the chalk 
after the action of acid. Many of the small lumps of the white granular 
material seemed to be intact, but the colloid silica having been removed, 
and with it probably a certain amount of terrigenous matter, these masses 
were now clear and translucent, almost invisible when mounted in balsam 
and viewed by transmitted light; with crossed nicols they were seen to 
consist of minutely crystalline chalcedony. A large number of the 
Foraminifera, cells, shell fragments, Radiolarians, and residuary sponge 
spicules were also found to be of chalcedony. The white granular material 
above described clings persistently to the spicules and other organisms 
contained in the rock, making the investigation of the Microzoa difficult. 
Only Glohigerina hulloides and Textularia minuta were isolated, though 
other forms can be seen in the section. The assemblage of spicules was 
similar to that in the specimen next to be described. 
A quantity of about 10 grams of fresh material was taken for the 
determination of the amount of lime and colloid silica by the method already 
alluded to. Nearly half the acid residue, or 19’31 per cent, of the entire 
rock, proved to be silica in the colloid state. There seemed but few 
mineral grains in the final residue, but it was not possible to estimate 
the quantity of terrigenous matter. 
A 8. — In this very curious specimen the recognisable elements are in 
large proportion to the mass of rock, but the facies is difierent from that 
of the last. There are but few shell fragments; Foraminifera, cells, or 
“ spheres ” form 35 per cent, of its mass ; sponge spicules and Radiolaria 
are very abundant. The matrix consists of fine calcareous matter mixed 
apparently with some terrigenous material ; there seems also to have been 
a certain amount of secondary crystallisation of the original calcareous 
ooze after deposition, and this masks to some extent the details in the 
structure of the more minute particles. When a little fine powder of the 
rock is examined, one can still recognise amongst it pieces of shell, of 
Foraminifera, pseudococcoliths, and the like. It is not possible to realise, 
as in the last specimen, that much colloid silica exists in the matrix ; a few 
