LOWER ROCKS AND GROUPS. 
33 
hundreds of miles in extent. Changes in the lithological features of a rock, also, which may 
render observations unsatisfactory or doubtful, are usually accompanied by greater or less 
change in the nature of the fossils. In no case, therefore, are to be overlooked either of the 
three important facts and characters, viz. Lithological character, order of superposition, and 
the nature of the contained fossils. A more extended knowledge of the development of the 
great groups of the older sedimentary deposits, which with some exceptions or interruptions 
extend almost across the continent, will enable us to appreciate more fully the comparative 
importance of the distinctive characters by which they are identified. 
Geol. 4tii Dist. 
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