220 JUHA. QUAHTZ ROCK. 



substances as in the alternations of micaceous schist with 

 quartz rock ; but in others, they are separated by a precise 

 line. Of this latter variety, Jura and Scarba offer striking 

 examples ; the quartz rock possessing a well-marked and 

 independent character, and forming a principal member 

 of the whole ; while the clay slate and micaceous schist 

 alternate with it. Among these three substances there is 

 consequently no decided standard of priority or poste- 

 riority ; the irregular recurrence of the whole proving that 

 each is, in its turn, the most ancient and the most recent. 



As limestone is found to alternate with micaceous schist, 

 it must sometimes possess the same relation to quartz rock, 

 and it accordingly occurs in this situation in the ridge 

 that forms Ben Gloe. No gradation however between the 

 two substances has yet appeared, if we except the slightly 

 indurated and siliceous nature of the limestone where they 

 are in contact. 



A more interesting alternation is that of quartz rock 

 with graywacke schist, and in this instance its analogy 

 with the secondary sandstones which so often follow that 

 substance, is particularly apparent. Jura, Scarba, and 

 Lunga, afford conspicuous examples of this association ; 

 but I need not repeat that which has so recently come 

 under review. Sutherland produces numerous and obvious 

 examples of the alternations between quartz rock and 

 gneiss; the two rocks occupying extensive tracts of 

 country, and forming some of the principal mountains of 

 the north-western coast. This substance alternates also 

 with the red primary sandstone, both of that district and 

 of Sky, as I have formerly remarked in treating of the 

 Sandstone islands. 



It has been imagined that quartz rock also belonged to 

 the secondary strata, because a compact stratified quartz is 

 found accompanying the beds of coal in Fife. But these 

 strata are intermixed with trap, and it was shown that in 

 Sky the secondary sandstones often put on that character 

 in similar situations. There is little reason therefore to 



