LOWEST RED SANDSTONE. 391 



source of considerable difficulty to geologists. 

 They have also been found in contact even with 

 the primary strata, as the superior secondary strata 

 of other kinds occasionally are ; and, in such 

 cases, it is evident that the difficulty may be 

 materially increased, and that all the sagacity of 

 the geologist may sometimes be required to make 

 the distinction, and to avoid being misled by the 

 consequences that would result, from an error in 

 this case, with regard to the succession of the 

 secondary strata. 



Although the mineral distinctions of these two 

 rocks, may be sufficiently determinate to enable 

 an observer to decide on their nature, when a 

 considerable number of specimens from any given 

 tract are examined, or when an average of the 

 characters of the whole deposit is taken, it will 

 not, in most instances, be safe, or even possible, 

 to draw such conclusions from the examination 

 of a small number, much less from that of a 

 single specimen. Similar cases, however, are of 

 such frequent occurrence in examining other 

 rocks, both among the primary and secondary 

 strata, that they offer no novelty in this particular 



