IN ROCKS OF DISSIMILAR ORIGIN. 333 
commonly present rounded angles or occur merely in rounded pellets 
or grains; Lewertophyrs, in which the leucite crystals sometimes 
give almost or quite circular sections ; Granites, in which the felspar 
erystals are sometimes rounded; Quartz-porphyries, in which the 
quartz frequently occurs in rounded grains; Porphyrites, in which 
quartz sometimes occurs in rounded grains similar to those in quartz- 
porphyries ; Porphyritic pitchstones and other vitreous rocks, in 
which the felspars and other porphyritic crystals often present 
rounded angles; Volcanic ejectamenta, in which rounded crystals are 
of common occurrence. 
Among Sedimentary rocks, Grits and Sandstones commonly consist 
in great part of crystals of quartz and felspar with rounded angles ; 
but these minerals are more often completely rounded, so that they 
frequently show no trace of crystalline faces. 
The porphyroids sometimes contain crystals which present per- 
fectly rounded contours ; but it has been shown, as already mentioned, 
that, in some cases at all events, their rounded contours are due to 
aborted crystallization. The Tiree limestone contains coccolite, 
rounded grains of augite. 
The Folkestone beds of the Lower Greensand contain rounded 
grains of glauconite; but, as already mentioned, the form of these 
grains is, at all events in some instances, due to the glauconite 
haying been deposited in the interior of Foraminiferal tests &c. 
Among the metamorphosed rocks, garnets frequently occur in 
rounded forms which evince no external trace of crystallization. 
The minerals chondrodite and pargasite also occur in roundish 
grains in granular limestones. 
Whether a connexion exists between any of these rounded forms 
(excluding, of course, those rounded by attrition) and the curvature 
of the faces of certain crystals, is a problem which may possibly 
deserve attention. 
In the Cubie system we have examples of 
Curvature in Rounding in 
Diamond. Garnet. 
Leucite. 
In the Rhombic system, 
Curvature (?) in Rounding in 
Olivine. Olivine. 
Chondrodite. 
In the Hexagonal system, 
Curyature in Rounding in 
Dolomite, Quartz. 
Chalybite, 
Corundum. 
In the Monoclinic system, 
Curvature in Rounding in 
Selenite. Augite, 
Orthoclase. 
Note. In this table the word curvature is used to signify the curvature of 
definitely developed faces of crystals, accompanied, of course, by curvature of 
the edges formed by the junction of such faces ; while the word rounding denotes 
abrasion of the surfaces of a crystal, which tends to obliterate edges and angles 
and to reduce the erystal to a spherical or spheroidal form. 
