636 STRATIGRAPHICAL GEOLOGY. [Boox VI. 
preceded geology in the order of birth—Chalk, Oolite, Greensand, 
Millstone Grit. Others are topographical, and bear witness to the 
localities where formations were first observed, or are typically 
developed—Oxfordian, Portlandian, Kimeridgian, Jurassic, Rheetic, 
Permian, Neocomian. Others are taken from local English 
provincial names, and remind us of the special debt we owe to 
William Smith, by whom so many of them were first used—Lias, 
Gault, Crag, Cornbrash. Others recognize an order of superposition 
as already established among formations—Old Red Sandstone, New 
Red Sandstone ; while still another class is founded upon numerical 
considerations—Dyas, Trias. By common consent it is admitted that 
names taken from the region where a formation or group of rocks is 
typically developed, are best adapted for general use. Cambrian, 
Silurian, Devonian, Permian, Jurassic, are of this class, and have 
been adopted all over the globe. 
But whatever be the name chosen to designate a particular 
eroup of strata, 1t soon comes to be used as a chronological or 
homotaxial term, apart altogether from the lithological character 
of the strata to which it is applied. Thus we speak of the Chalk or 
Cretaceous system, and embrace under that term formations which 
may contain no chalk; and we may describe as Silurian a series of . 
strata utterly unlike in lithological characters to the formations in 
the typical Silurian country. In using these terms we unconsciously 
adopt the idea of relative date. Hence such a word as Chalk or 
Cretaceous does not so much suggest to the geologist the group of 
strata so called, as the interval of geological history which these 
strata represent. He speaks of the Cretaceous, Jurassic, and Cam- 
brian periods, and of the Cretaceous fauna, the Jurassic flora, the 
Cambrian trilobites, as if these adjectives denoted simply epochs of 
geological time. 
The Geological Record is classified into five main divisions :—(1) 
the Archean, sometimes called Azoic (lifeless), or Hozoic (dawn of 
life); (2) the Paleozoic (ancient life) or Primary; (8) the 
Mesozoic (middle life) or Secondary ; (4) the Cainozoic (recent 
- life) or Tertiary, and (5) the Post-Tertiary or Quaternary. ‘These 
divisions are further ranged into systems, each system into series, 
sections, or formations, each formation into groups or stages, and 
each group into single zones or horizons. The following generalized 
table exhibits the order in which the chief subdivisions appear. 
