JURASSIC HISTORY. 
3ii 
A. M. DAVIES. 
In the July number we noted the appearance of Part XXXII. 
of Mr. S. S. Buckman’s Type Ammonites, in which some 
Yorkshire species were figured. Three more parts have 
since appeared, one of which (Part XXXIV.) contains a 
contribution to the subject of Jurassic chronology that should 
be read by everyone interested in the reconstruction of the 
conditions and events of past geological periods. 
Mr. Buckman’s work is so often misunderstood, that it 
may not be out of place to point out that his ideas are funda- 
mentally those of William Smith, only applied with greater 
precision. The new ideas with which he himself has sup- 
plemented them are principally three. Firstly, that of 
homoeomorphy, or the close superficial resemblance of species 
of diverse origin and often distinct age, which makes hasty 
field determination dangerous. Secondly, the idea that the 
sediment accumulated in any area during a given time is 
an algebraic sum of deposition and erosion, an idea which 
invalidates much easy correlation based on the notion that 
beds in two areas must be synchronous because the beds 
above or below them (or both) are synchronous. Thirdly, 
the distinction (in the case of ammonites) between drifted 
shells and shells buried in their natural habitat, a distinction 
of vital importance in any "attempt at recognizing ancient 
life-provinces. While fully admitting that such provinces 
can sometimes be defined, Mr. Buckman argues that many 
apparent cases are due to the mistake of taking faunas as 
synchronous which are not. His actual suggestions of 
Jurassic geography will be found in Part XXXIV. He also 
gives a provisional tabulation of the Jurassic period, dividing 
it into 43 ages, each characterized by the dominance of some 
one Ammonite genus after which it is named (each age is in 
turn composed of a number of hemerse, but these are not 
now dealt with ; they have been partially tabulated in earlier 
parts) . 
Two questions naturally suggest themselves. Firstly, was 
the Jurassic a typical or an exceptional period in the earth’s 
history ? In estimates of geological time based on thicknesses v 
of strata we generally notice that the Jurassic system appears 
very thin for its admitted importance. These estimated 
thicknesses do not make the allowance that Mr. Buckman 
insists upon for unequal accumulation and quasi-contem- 
poraneous erosion : his own estimates of maximum thickness 
would be many times larger. But do all systems require the 
same factor of enlargement? Seeing that the most fully 
1922 Oct. 1 
