Dr. M'Culloch's Supplementary Remarks on Quartz Rock. 57 



quartz rock is known to myself, the materials from which a general account 

 of its characters and geological connexions may be deduced. 



In a very extensive review of the north-western coast of Scotland^ and 

 among the rest of that tract described in the preceding paper, the observations 

 which it contains have been both confirmed and multiplied ; — quartz rock 

 having been found to extend over considerable spaces in Ross-shire, in places 

 which I had not visited in the summer of 1817. In these it occurs in alter- 

 nation with gneiss, as well as with that red sandstone, which, from its situation 

 both in Sky and on the main land, I have thought fit to rank among the pri- 

 mary rocks. In these cases there is sometimes an obvious gradation between 

 ihe. latter and the quartz rock ; but in many places the distinction between 

 the two is strongly marked. In the predominant examples, however, it appears 

 to alternate with gneiss, and often in beds, or in a series of repeated beds of 

 equal dimensions ; and this distinction between the two rocks is here also 

 marked almost invariably with great precision. 



The entrance of the eastern side of Loch Eribol consists entirely of quartz 

 rock ; and it is here very remarkable for the magnitude, number, and 

 variety of the caves which it contains, and for the singular and picturesque 

 forms of various kinds into which it is broken. 



Here, as at Macarthur's Head, in Isla, the quartz contains pyrites in consi- 

 derable quantity ; and the effect produced by the decomposition of this latter 

 substance, is such as to be very deceptive; the exterior surface of the cliffs of 

 quartz rock being frequently stained of a reddish brown hue, so as no longer 

 to resemble that substance, so generally conspicuous by its whiteness ; but 

 very exactly imitating the colour of granite. The deception is often rendered 

 perfect by the forms of the rock, and can only be detected by manual exami- 

 nation : and it will be useful to have pointed out this circumstance ; since, 

 either from confidence in the supposed facility of determining the characters 

 of rocks at a distance, or from the impossibility of obtaining a near access, 

 geologists of confined experience may easily be led by it into erroneous con- 

 clusions. 



In Loch Eribol the quartz rock also presents examples of considerable 

 curvatures, which, in general, is a very rare occurrence ; as, in most cases, 

 where sudden flexures or contortions occur in the micaceous and argillaceous 

 schists by which it is accompanied, it is itself fractured and not bent. Near 

 the Whiten head, in this inlet, a remarkable example of this limited curvature 

 may be seen ; the exterior parts having fallen away so as to leave a large por- 

 tion exposed in the form of a semicylindrical body. 



In a former paper it was remarked, that a considerable tract in Mar 



VOL. Tl. I 



