in the South English Chalk. 



363 



that these were formed in cracks after the chalk had consolidated, it is 

 obvious that they have no necessary connection vrith bedding planes, 

 however much they may tend to follow them, nor can their 

 impersistency be relevant to interstratified lines of flints. At the 

 same time marl seams as a rule are more constant than lines of flints 

 in their individual characters : that is to say, a marl seam will retain 

 the same degree of thickness and definiteness over a long distance, 

 while lines of flints will often vary from wide and closely set to thinly 

 strung out in the course of a few feet. Marl seams are therefore 

 much more useful than flint lines in tracing the effect of the 

 dislocations which are so often met Avith. I also regard them as 

 no less persistent and as a rule more persistent than flint lines 

 over long distances. An admirable instance is to be found in 

 the sections under discussion in the marl bed I have called the 

 *' 2 inch Marl ", This bed consists of from 2 to 3 inches of solid 

 marl, and forms a conspicuous feature in the cliff and a ready 

 guide to the general position of the upper boundary of the subzone ; 

 it comes in just above the Roedean School tunnel and occurs with 

 perfect regularity right away to Seaford Head. I have, therefore, on 

 all three grounds of constancy in thickness, definiteness, and 

 persistency, come to attach the chief importance among lithological 

 features to marl seams. 



