TO STRATIFIED ROCKS. 39 
from a sense of the convenience of this long re- 
ceived arrangement, than from the reality of 
any strongly defined boundaries by which the 
strata, on the confines of each series, are sepa- 
rated from one another. 
As the materials of stratified rocks are in great 
degree derived, directly or indirectly, from those 
which are unstratified,* it will be premature to 
enter upon the consideration of derivative strata, 
until we have considered briefly the history of 
the primitive formations. We therefore com- 
mence our inquiry at that most ancient period, 
when there is much evidence to render it 
probable that the entire materials of the globe 
were in a fluid state, and that the cause of this 
fluidity was heat. The form of the earth being 
that of an oblate spheroid, compressed at the 
poles, and enlarged at the equator, is that which 
a fluid mass would assume from revolution 
round its axis. The further fact, that the short- 
est diameter coincides with the existing axis 
* In speaking of crystalline rocks of supposed igneous origin 
as unstratified, we adopt a distribution which, though not strictly 
accurate, has long been in general use among geologists. Ejected 
masses of granite, basalt, and lava have frequently horizontal 
partings, dividing them into beds of various extent and thickness, 
such as those which are most remarkable in what the Wernerians 
have called the Floetz trap formation, PI. 1, section Fig. 6. ; but 
they do not present that subdivision into successions of small beds, 
and still smaller laminse, which usually exist in sedimentary 
strata that have been deposited by the action of water. 
