224 THE GEOLOGICAL STRUCTURE OF THE 



that eyen in a short distance, clay passes laterally into sand 

 or limestone. Therefore a map drawn on the stratigraphical 

 method calls the same fossiliferons horizon by different 

 names, in other words assigns to it different dates in dif- 

 ferent places, to the great confusion of the palaeontological 

 student. Such a non-agreement between the palaeontological 

 and the stratigraphical methods obtains at Dundry com- 

 pared with surrounding areas ; and therefore tlie divisions 

 accepted by us in our mapping are based upon purely 

 palaeontological lines. 



If our method of mapping were extended to other dis- 

 tricts the result would be the production of a chronological 

 map based on palaeontological evidence, — a map which 

 showed the exact date of the different deposits according 

 to their fossil evidence instead of a map which often shows 

 only where one sedimentary condition ended and another 

 began. Of course, in certain cases a palseontological map 

 would do the same : thus, at Dundry our line marking the 

 commencement of deposition of Aalenian date practically 

 coincides with the upper limit of clay ; but if this line were 

 to be extended to the Frocester district of the Cotteswolds 

 it would be some 250 feet above the clay limit. Therefore 

 in digging a well at Dundry the finding of a Diimortieria 

 would show that the water level was reached ; but the 

 finding of the same fossil in the Frocester district of the 

 Cotteswolds would indicate the hopelessness of trying for 

 water. The geological information would say " go 250 feet 

 lower down." 



The divisions which we have accepted are as follows ; and 

 in order that they may be compared with the divisions 

 adopted in their maps by the Survey, we place the two side 

 by side. 



