xxxvi 
INTRODUCTION 
work on Yorkshire Geology for information with reference to 
the source of the pebbles in Coal Measure sandstones. The only 
way (except reading it), of asceitaining where such inform- 
ation occurred in the volume, was to look under the word 
' sand-stone,' which was indexed as appearing on some dozens 
of pages. To all of these, therefore, reference had to be made 
in order to obtain the necessary information. It seems clear 
that there ought to have been an index to the ' sand-stone ' 
entries, or, in other words, an index to the index. The necessity 
for giving every such detail in the following index, accounts 
for its unusual length. 
