ROCKS AND STRATA. 19 
on this branch of Geology. For the convenience of the 
general reader I subjoin a synoptical view of the characters 
and relations of the British fossiliferous deposits. 
The total thickness of the entire series of rocks within 
the scope of human examination, is estimated at from fifteen 
to twenty miles, reckoning from the summits of the highest 
mountains to the greatest depths hitherto penetrated ; and 
as this vertical section scarcely amounts to ;1,th of the dia- 
meter of the globe, it is familiarly termed the Earth’s crust. 
The substances of which the sedimentary strata are composed 
have been deposited by the action of water, and subse- 
quently more or less modified in structure and composition 
by heat, and by electro-chemical forces. The superficial 
accumulations of water-worn detritus, consisting of gravel, 
boulders, sand, clay, &c. are termed Drift, or Alluvial depo- 
sits. When the successive layers in which the sediments 
subsided are obvious, the deposits are said to be stratified ; 
when the nature of the materials has been altered by igneous 
action or high temperature, but the lines of stratification 
are net wholly effaced, the rocks are denominated metamor- 
phic (transformed). When all traces of organic remains 
and of sedimentary deposition are lost, and the mass is 
crystalline, and composed of known products of igneous 
action, such rocks are named plutonic, as granite, sienite, 
trap, basalt, porphyry, and the like. Lastly, rocks resem- 
bling the lavas, scorie, and other substances emitted by 
burning mountains still in activity, are called volcanic. 
The sedimentary origin ascribed to ancient crystalline 
rocks is, of course, hypothetical, since all evidence of aqueous 
deposition is wanting, and the minerals (mica, quartz, and 
felspar) of which they are so largely constituted, are not 
readily soluble in water under ordinary circumstances. But 
rocks unquestionably deposited by water, when exposed to 
intense heat under great pressure, acquire a crystalline 
Structure (Wond. p. 864) ; and a series of changes, from a 
