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POPULAR SCIENCE REVIEW. 
rocks, and, for that reason, might, as before mentioned, not 
inappropriately be termed derivate rocks. When found in the 
normal state of sedimentary deposition, they may be conveniently 
subdivided into — 
1. Eocks formed of the immediate products of the breaking- 
up of eruptive rocks. 
2. Eocks built up of the more or less rounded or angular 
debris of previously existing sedimentary or eruptive 
rocks. 
3. Eocks composed of mineral substances extracted from 
aqueous solution by crystallisation, precipitation, or the 
action of organic life. 
1. Rocks composed of the immediate products of the breaking 
up of eruptive rocks. — The little attention paid by geologists in 
general to the study of rocks of this class, has introduced the 
elements of confusion into many of their enquiries, and frequent!} 7 
has led to very erroneous opinions being formed as to the nature 
and origin of certain rocks, which could never have been enter- 
tained had microscopic investigation gone hand in hand with 
field observation. 
Eocks of this class may either be of subaerial or subaqueous 
origin; in the former case, for example, volcanic ashes may 
have been deposited as beds on the surface of the land, and 
afterwards been covered by lava streams poured out over them ; 
or, from having been depressed below the sea level, may have 
had sedimentary beds of aqueous origin subsequently superposed 
on them. 
Yv 7 hen of subaqueous origin, as is by far the most common 
case, subaerial or subaqueous outbursts may force into the sea 
eruptive rocks, which, being at once broken up into a state of 
division, more or less fine, in proportion to the greater or lesser 
cooling power of the water mass in immediate contact, may be 
spread out into beds by the action of the waves : the texture of 
these rocks may vary from that of the coarsest breccia down to the 
finest mud, and, as is usually the case, such deposits may present 
themselves as alternating beds of coarse and fine character. 
Upon the consolidation of such formations, rocks are formed, 
identical in chemical and mineralogical composition with the 
original eruptive rock from which they were derived, and which, 
particularly when close-grained, often present an external ap- 
pearance so like the original rocks as to be frequently undis- 
tinguishable from them by the naked eye ; in such deposits it is 
often easy to pick out specimens having all gradations in appear- 
ance from the above described down to such as would be attri- 
buted to the consolidation of mere detrital mud. 
No wonder, therefore, if the field geologist finds himself 
