302 ¥F, RUTLEY ON COMMUNITY OF STRUCTURE 
by infiltration. 
Pseudomorphs ... {Y replacement. 
by alteration.. ee oer 
if psephitic (breccias and conglomerates). 
Clastic matter...... psammitic (sands). 
| pelitic (muds). 
animal. 
Organic remains... { vegetable. 
After this short réswmé of the leading structural characters which 
rocks present, it may be well to follow it up by a consideration of 
certain rocks, or groups of rocks, in which structural peculiarities 
may be regarded as indicative of their origin, and others in which 
structural ‘hesnchens are of no value in elucidating the conditions of 
their genesis. 
Rock-forming materials. 
( Crystals with sharp angles. 
5 with rounded angles. 
| ie with no angles (aborted crystallization). 
Fragments of erystals with sharp angles. 
| a with rounded an gles. 
| C) ~ystalline grains of irregular and angular form, as in 
granites. 
C ~ystallites. 
Fragments of Rocks with sharp angles. 
ae with rounded angles. 
Segr egations, Concretions, &e. 
Solids, fluids, vapours or gases filling, or partially filling, 
cavities in minerals and rocks(amygdaloids, fluid-cayities, 
t steam-pores, &c.). 
Solids precipitated from infiltered solutions and constitu- 
{ 
| 
Common to Erup- | 
tive and Clastic 4 
rocks. 
ting cements. 
Solids precipitated from solution, constituting rock masses 
entirely or partly. 
Solids secreted from solution and from other sources by 
organic agency, forming the tests of Foraminifera, Mol- 
lusks, &c., and the bones of vertebrate animals. 
Peculiar to Erup- { Amo “phous glassy matter, resulting from the rapid cooling 
tive rocks. of fused mineral matter. 
Peculiar to Clastic { Débris water-worn 
rocks. 1, unwater-worn 
Peculiar to Clastic 
rocks. 
} cemented and uncemented. 
In the preceding table I have treated all rocks under two heads, 
the Eruptive and the Clastic. 
The latter may be divided into :— 
Ordinary Clastic rocks or ordinary detrital sediments, and Pyro- 
clastic rocks or volcanic ejectamenta. 
As examples of eruptive rocks containing crystals with sharp angles 
may be cited :—Buasalts, in which the augite, felspars, nepheline, 
magnetite, &c. occur in well-developed orystals; Phonolites, in which 
the nepheline and other constituents are commonly well crystallized ; 
Granites, in which the felspars, apatite, and often the micas are met 
with in well-formed crystals. 
As instances of eruptive rocks containing crystals with rounded 
angles we may quote :—Basalts, in which the olivine crystals very 
