PRESTWICH — WOOLWICH AND READING SERIES. 11 



mineral or palEeontological characters alone. It is this feature which 

 forms one of the chief points of interest of the group, for if it is im- 

 portant to identify strata by their organic remains or by their litho- 

 logical structure, it is not less so to trace the changes of composition 

 which can occur in strata on the same plane, to note the modifications 

 in the fauna by which such changes are accompanied, and to deter- 

 mine the limits to which the variations may extend. The case now 

 before us is, so far as it regards the dimensions of the deposit itself, 

 one comparatively of small importance, but it is valuable from the 

 clear and unmistakeable testimony which it affords on these points. 

 It was the extremely variable character of this group, which putting 

 on occasionally the appearance of the group beneath, and at other 

 times assuming the character of the one above it, that led to the im- 

 pression of a want of order and of irregularly recurring strata through- 

 out the whole of the Lower Tertiaries. So deceptive, indeed, are 

 these common points of structure, that it is only lately that I have 

 been able to satisfy myself that these changes are confined essentially 

 to one portion of the series, and that one restricted to the limits of 

 the middle division, and that strata so very dissimilar are really 

 equivalent. This once determined, and having eliminated the two 

 more uniform groups, it becomes apparent that there is in the 

 "Lower London Tertiaries" a defined order of superposition formed 

 of three distinct and independent groups of strata. 



At the same time there cannot well be strata varying more in ap- 

 pearance and character than we find forming this series in the sepa- 

 rate sections at Reading, Deptford, Blackheath, and Heme Bay ; by 

 tracing the group at short intervals it is seen that it is by actual altera- 

 tion in some of its beds, effected as they range from west to east, as 

 much as by the thinning out of others, that these changes are pro- 

 duced. The strata are, in fact, on the same horizon and clearly svnchro- 

 uous (see PI. I. Diag. A & C). Under these circumstances there are 

 objections to giving this division a simple designation dependent either 

 on mineral character or on place, for the former is constantly varying, 

 and the type of the series in one district may be entirely difl'erent 

 in another. Still a name taken from some well-known place is probably 

 the least objectionable, or rather the more convenient, and I purpose 

 therefore to term this division the " Woolwich and Reading series," 

 as the two principal forms of structure are well exhibited in the sec- 

 tions at and around these localities*. I shall, however, in speakino- 

 of this group, sometimes use the name of that locality only, to the 

 particular characters of which the observations may have reference. 



The grouping of the "Lower London Tertiaries" will therefore 

 stand thus : — 



I. The Basement-bed of the London Clay. 

 11. The "Woolwich and Reading Series. 

 III. The Thanet Sands. 



* The Woolwich section has the inconvenience of exposing the three divisions 

 of the " Lower London Tertiaries " together. The term apphes only to the niiddle 

 part of the section (see Pi. \. Diag. A. Loc. sect. 25). 



