70 Introduction to Geology. 
of tertiary strata are developed under the most interesting 
circumstances. The present is not the time or place for ad- 
verting more particularly to the extraordinary local disturbance 
by w hich so many of the superior formations have acquired 
a vertical position. 
General Observations on the Strata which contain Organic 
Remains. 
The word Formation is understood to apply to certain groups 
of strata which have been associated together for the sake of 
ready and simple classification, and of avoiding the perplexity 
otherwise arising from their numerous subordinate beds. But 
for this comprehensive arrangement, it would be difficult to 
distinguish, with sufficient precision, the separate members of 
the secondary strata. As each formation is, in general, made 
up of several members, so also the latter are composed of an 
infinite number of beds ; and those beds, again, of imumerable 
finer lamina: but the w hole series being found to possess some 
common character, some obvious relations, or some general 
similarity in organic accompaniments or circumstances of po- 
sition, they are appropriately classed under one common deno- 
munation. 
In this country these names are chiefly derived from some 
predominant quality or structure; as the green or chloritic 
sand, ferruginous sands, red sandstone, magnesian or dolomitic 
limestone, gault, clunch, and oolites. ‘To some of these de- 
scriptive titles others have been superadded, which are derived 
from certain localities where these formations are best de- 
veloped; such as the Felkstone marl, Weald clay, Oxford 
clay, Hastings sands, Woburn cane and Shanklin sands. 
Others are wholly derived from localities ; as the London clay, 
Kimmeridge clay, Portland limestone, Petworth marble, Forest 
marble, &c. 
As the formations comprehend many inferior beds, it must 
often occur that they contain some whose mineralogical cha- 
racters differ materi ially from those imported by their names. 
For this reason a nomenclature derived from characters not 
universal must be considered defective. Of this class is what 
is termed the plastic clay formation, which chiefly consists of 
sand. The Weald clay contains thick beds of sand, with lime- 
stone and fresh-water marbles. We may add also the clays 
and limestones of the ferruginous sands, the Kelloways rock, 
the Oxford clay, &c. 
The practical geologist is often led to perceive frequent 
transitions and modifications 1 in the structure of the same beds ; 
the clays often changing to indurated argillaceous rocks, shales, 
