214 LOWEST, OR OLD RED, SANDSTONE. 



even where the geological analogies are unques- 

 tionable. It is therefore his duty to study these diffe- 

 rences, without allowing himself to be misled by hy- 

 pothetical identities. Time will thus produce a more 

 perfect history of this rock ; while I can now only 

 describe its characters as they have fallen under my 

 own inspection. To borrow from other observations, 

 would but incur the risk of confirming the present 

 confusion ; while an apparent path is often willingly 

 trod till it is beaten, even where it misleads. 



If it is not always possible to distinguish these 

 three red sandstones, except by careful geological in- 

 vestigations, since the mineral distinctions give but 

 limited assistance, the alternation of the primary sand- 

 stone with gneiss or other primary strata, is an infal- 

 lible geological criterion for that rock. With respect 

 to the red marl, the presence of salt is equally infallible ; 

 as that of gypsum is a good test, if not absolute. Its 

 superiority to the coal series is another, as is its imme- 

 diate inferiority to the Lias limestone. Although the 

 old red sandstone is sometimes sufficiently distinct 

 from this in mineral characters, it is not always so ; 

 and I do not therefore offer it to the student, to whom 

 the more infallible test of its geological connexions is 

 recommended. 



The sandstone under review appears to be one of 

 the most generally diffused rocks in nature ; and may 

 thus be considered, like gneiss, among the deposits 

 commonly called universal. Occupying extensive 

 spaces, it also forms masses of considerable thickness, 

 producing mountains of conspicuous elevation. Foula, 

 in Shetland, attains to the height of 1400 or 1500 feet ; 

 Keacloch in Rossshire to nearly 4000. It is so uncertain, 

 however, whether many of the foreign examples belong 

 to this or to the red marl, that I dare not quote them 



