336 ¥, RULLEY ON COMMUNITY OF STRUCTURE 
(1) Volcanic ejectamenta (ashes, dust, sand, and lapilli) consist 
of fragments of minerals and fragments of rocks. These fragments 
are sometimes angular, at other times they exhibit rounded con- 
tours. Besides these fragments they often consist to a greater or 
less extent of entire crystals, which may also have their angles and 
edges either perfectly sharp or else rounded. In all cases the 
rounding is due either to attrition or fusion, or to both of these 
causes. 
(2) Sedimentary rocks (especially grits, sandstones, and tufts) 
consist of fragments of minerals, and sometimes fragments of rocks. 
These fragments may be either angular or rounded. Sedimentary 
rocks also frequently contain entire crystals, which may also have 
their angles and edges either perfectly sharp or else rounded. In 
all cases the rounding is due to attrition, except in crystals whose 
development has been aborted, or unless actually fused volcanic 
ejectamenta have become incorporated with the sediments. It is 
possible that in some cases sandstones may result from actual 
crystallization from solution; and the formation of flint may be 
regarded as due to a similar origin. A felspathic grit formed under 
such conditions may not be distinguishable from a felstone either 
structurally or mineralogically. From these considerations we may 
assume that, structurally and mineralogically, as flint is to sandstone 
so is felstone to felspathic sandstone ; and since flint appears to be an 
intimate admixture of crystalline and amorphous matter, the polari- 
zation-picture of flint closely resembles that of some forms of 
petrosilex or felstone. 
(3) The minerals which constitute volcanic ejectamenta are the 
same as those which enter into the constitution of lavas or other 
eruptive rocks, the former being usually derived either in great part 
or entirely from the latter. 
These minerals are principally felspars, augite, hornblende, olivine, 
micas, and quartz. 
(4) The minerals which constitute sedimentary rocks are chiefly 
quartz, felspars, micas (in rocks of the grit, sandstone and tuff types), 
clays partly resulting from decomposition of felspars, serpentinous 
matter, chlorite, &c. resulting from the decomposition of augite, stea- 
tite, tale, hornblende, olivine, &c. 
(5) The minerals occurring in the two classes of rocks may there- 
fore be classed as follows :— 
Volcanic ejectamenta and Sedimentary rocks, 
other eruptive rocks. Grits, Sandstones, Tuffs, &e. « 
Felspars. Felspars and decomposition-products from felspars. 
Augite. Decomposition-products from augite. 
Hornblende. Fe Bs » hornblende. 
Olivine. 5 3 > Olivine. 
Micas. Micas. 
Quartz. Quartz. , 
Clays, shales, and slates may consist of materials 
derived from the decomposition of felspars, usually 
with admixture of more or less quartz, and some- 
times mica or tale, &e. 
