DIVISION OF THE LIAS. 
353 
CHAPTER XX. 
JURASSIC GROUP— Continued, —lias. 
Mineral Character of Lias.—^Numerous successive Zones in the Lias, marked 
by distinct Fossils, without Unconformity in the Stratification, or Change 
in the Mineral Character of the Deposits.—Gryphite Limestone.—Shells 
of the Lias.—Fish of the Lias.—Reptiles of the Lias.—Ichthyosaur and 
Plesiosaur.—Marine Reptile of the Galapagos Islands.—Sudden Destruc¬ 
tion and Burial of Fossil Animals in Lias.—Fluvio-marine Beds in Glou¬ 
cestershire, and Insect Limestone.—Fossil Plants.—Origin of the Oolite 
and Lias, and of alternating Calcareous and Argillaceous Formations. 
Lias. —The English provincial name of Lias has been very 
generally adojDted for a formation of argillaceous limestone, 
marl, and clay, which forms the base of the Oolite, and is 
classed by many geologists as part of that group. The pe¬ 
culiar aspect which is most characteristic of the Lias in Eng¬ 
land, France, and Germany, is an alternation of thin beds of 
blue or gray limestone, having a surface which becomes light- 
brown when weathered, these beds being separated by dark- 
colored, narrow argillaceous partings, so that the quarries of 
this rock, at a distance, assume a striped and ribbon-like ap¬ 
pearance. 
The Lias has been divided in England into three groups, 
the Upper, Middle, and Lower. The Upper Lias consists first 
of sands, which were formerly regarded as the base of the 
Oolite, but which, according to Dr. Wright, are by their fos¬ 
sils more properly referable to the Lias; secondly, of clay 
shale and thin beds of limestone. The Middle Lias, or marl- 
stone series, has been divided into three zones; and the Lower 
Lias, according to the labors of Quenstedt, Oppel, Strickland, 
Wright, and others, into seven zones, each marked by its own 
group of fossils. This Lower Lias averages from 600 to 900 
feet in thickness. 
From Devon and Dorsetshire to Yorkshire all these divis¬ 
ions, observes Professor Ramsay, are constant; and from top 
to bottom we can not assert that anywhere there is actual 
unconformity between any two subdivisions, whether of the 
larger or smaller kind. 
In the whole of the English Lias there are at present known 
about 937 species of mollusca, and of these 267 are Cephalo- 
pods, of which class’more than two-thirds are Ammonites, 
