RED PRIMARY SANDSTONE. 181 



quently be procured in parallel beds of various thick- 

 ness ; while, often possessing natural joints, like many 

 other schistose rocks, it breaks readily into fragments, 

 easily trimmed by the hammer. The difficulty of 

 producing a smooth surface by the pick, might limit 

 its uses to certain classes of masonry ; but in those, 

 the facility of giving it a form fit for building, would 

 render it an ceconomical substitute for granite, which 

 it also far excels in durability. As a lining for fur- 

 naces, its value is obvious. 



The want of foreign observations on this rock, pre- 

 vents us from knowing whether it is a repository of 

 metals. If it be excluded from the metalliferous rocks 

 because it contains no metals in Scotland, we might 

 draw the same conclusion with regard to many others. 

 Yet at Tyndrum, as noticed under micaceous schist, ga- 

 lena and blende are found in a hill which consists of al- 

 ternate beds of quartz rock and micaceous schist, and 

 in which the former is nearly as abundant as thelatter. 



Red primary Sandstone. 



I have associated this in the same chapter with 

 quartz rock, for reasons which will ultimately appear; 

 but before proceeding to its history, I must limit its 

 range as formerly given in the account of the Western 

 Isles. A re-examination of some of these districts has 

 shown, more clearly, what I then suspected ; that 

 some of the examples then quoted belong to the old 

 red sandstone ; the difficulty of 'distinguishing- them 

 where the secondary follows the primary one, espe- 

 cially in conformable order, being extreme, or almost 

 insurmountable. 



The varieties of this sandstone are very limited ; 

 nor does it require any further general definition than 

 that of the secondary red sandstone. It is, fundamen- 

 tally, an aggregate of quartz and red felspar, vary in :r 



