i 
STRATIFIED OR SEDIMENTARY ROCKS, 3l 
STRATIFIED OR SEDIMENTARY ROCKS. 

According to the different origin and mode of formation of these 
rocks they may be divided into three classes :— 
1. MECHANICAL Deposits, ' 
a, Accumulated and stratified by water (aqueous). 
b. Accumulated and stratified by wind (aerial). 
2. CHEMICAL PRECIPITATES. 
8. ORGANIC PROCESSES. 
a, From the growth and accumulation of animal matter (zoogenic). 
b. From the growth and accumulation of vegetable matter (phytogenic). 
The above three classes may again be divided, according to their 
lithological and mineral characters, into the five following groups :— 
1. Arenaceous or Siliceous ac ... Sand A 
2. Argillaceous or Aluminous‘ ... .-. Clay 
3. Caleareous ax nee ay .. Lime 
4. Ferruginous ... Aa ou: ... Iron 
5. Carbonaceous ... D st; . Coal, Lignite, &c. 
Nearly all sedimentary rocks are stratified, that is, they lie in beds 
or layers, one above the other, and often quite parallel for long distances. 
With few exceptions these beds or layers are made up of larger or smaller 
fragments and particles (debris) of pre-existing rocks, washed together 
and deposited from a state of suspension in water. A few only are the 
result of chemical precipitate of mineral substances from aqueous solution 
—gypsum, rocksalt and some limestones,—and these usually possess a 
crystalline ‘structure, not otherwise observed in “unaltered” rocks of 
sedimentary origin. Many contain organic remains (fossils), more or less 
distinct, while some are entirely composed of such. Most of these 
organisms are supposed to have lived and died in the water, at the 
bottom of which the sediment was being deposited, that now forms 
the rock they are now embedded in. Others, chiefly vegetable, have 
been washed or otherwise transported off adjoining dry land, or the 
land, on which they existed, has been submerged, and thus covered with 
sand, mud, or silt. 
Respecting the igneous or unstratified rocks, it has been stated, that 
no marked line can be drawn’ between the various classes and subdi- 
visions, as they are found, both in their physical and mineral characters 
and in their lithological relations, to merge into each other. This 
characteristic is also more or less common to the sedimentary and strati- 
fied rocks. The several sedimentary deposits have been divided into 
“formations” according to the order of their superposition, and conse- 
quently of their age, and these are gathered into four groups, repre- 
senting longer periods of deposit :— i 
‘1. Primary, or Paleozoic. 
2. Secondary, or Mesozoic, 
8. Tertiary, or Cainozoic, 
4. Post Tertiary, or recent. 
These divisions have no reference to mineral or lithological cha- 
racter, and no specimen of rock—such as sandstone, clay-slate, shale, 
