218 Notes on a Visit to the Southern Highlands of Scotland. 
sometimes many yards broad, in crystalline and fragmentary 
rocks. Gold is found associated with this variety. 
Granite consists of a granular crystalline mixture of quartz, 
felspar, and mica in varying proportions. Gneiss has a similar 
composition, but has a stratified appearance from the mica being 
in layers. These rocks are of igneous origin, and the constituent 
minerals have separately crystallized from the molten mass 
during slow cooling. 
In the sandstones we find the ruins of older rocks. The 
quartz grains of which they are composed are more or less firmly 
cemented together by various substances. When the cementing 
material is silica, the grains are often so firmly fixed in the matrix 
that a fracture passes through the individual grains instead of 
between them. Such a rock is quartzite. 
A silicious conglomerate is precisely similar in character to 
a sandstone, but the granules are large and usually rounded. 
Carnarvon Castle is built of a siliceous conglomerate similar 
to the piece exhibited, which was picked up on the shores of the 
Menai Straits. 
There are several slides of fossil wood exhibited. Under the 
microscope the cell-structure is perfect. The wood from Croydon 
was found imbedded in a flint. The wood must have floated 
about on the sea of the Cretaceous period for some time, as it 
was bored in several places by teredos. On sinking to the 
bottom, the woody structure was gradually replaced by silica, 
and finally embedded in a crust of the same material. 
Silica is very largely used in glass-making, and in the 
manufacture of china, either in the form of ground flints or of 
pure sand. 
I must no longer trespass upon your patience. There are 
several other siliceous minerals to be met with, but it will be 
found that they can be classed under the head of one of the 
foregoing varieties. The dividing lines are by no means always 
sharply defined, one form passing into another, even in the 
same specimen. 
131.—Novres on a Visit to THE SourTHERN HIGHLANDS OF 
ScorLanp. 
Mr. T. Doveras exhibited specimens of chert containing 
radiolaria from Crawford, Lanarkshire, N.B., in the Southern 
Highlands. The chert outcrops on the upper part of the Castle 
Hill, at Crawford. Mr. Douglas was informed by an Edinburgh 
geologist that the underlying basis of most of the hills in the 
neighbourhood was trap, and that superimposed on this were 
