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



consisted of quartz rock ; but I was not then aware of the exact mode in 

 which it occurred with respect to the other rocks with which it is there asso- 

 ciated. This whole district presents a perpetual alternation, both in the great 

 and small scale, of gneiss and micaceous schist, but principally of the former 

 substance, with the rock in question. And here, not only the alternation 

 between the gneiss and quartz rock is so frequent, but the apparent transition 

 between the two is so perfect, that it is quite impossible, without minute exami- 

 nation of the fractured rock, to determine which is present; the external aspect 

 and general characters of both being precisely the same. Partly from this fre- 

 quency of alternation, and partly from the impossibility of determining the na- 

 ture of the rocks without such minute investigation as is impracticable, it is 

 not possible, in a map, to express the nature of the rocky strata, except in a 

 general manner. 



The association of gneiss with quartz rock is less common in the northern 

 parts of Aberdeenshire ; but where the latter substance occurs there, in the 

 vicinity of granite, it is attended by appearances of an interesting nature, more 

 particularly as respects the history of that compound. Ben-na-chie, among 

 other places, presents a very remarkable and accessible example of the changes 

 of character which the quartz rock thus exhibits in contact with the granite. 

 The regularity of the stratification is disturbed, and in many places the strata 

 are also fractured and displaced. Where the fractures are considerable, the 

 parts are sometimes reunited by minuter fragments and by crystalline quartz ; 

 and in many places these fragments are so numerous, that the whole mass 

 forms a breccia or an angular local conglomerate of a peculiar character. In 

 other cases cavities remain in the intervals ; and their surfaces are then gene- 

 rally covered with minute drusy crystallizations of quartz. 



Many of these beds, also, lose the character of quartz, and put on the aspect 

 of chert, sometimes gray, and at others of a reddish, or red-brown colour. 

 Others, of a more argillaceous character, resemble jasper, and are indeed in 

 many places not to be distinguished from the most siliceous varieties of that 

 mineral ; nor is it uncommon to find an intimate mixture of quartz, chert, and 

 jasper in the same bed, sometimes disposed in an irregularly laminar manner, 

 at others intermixed in great confusion. 



If we recollect the variable composition of quartz rock where it occurs at a 

 distance from granite, it is not difficult to account for the peculiarities of cha- 

 racter in the instance under review. They are in all respects analogous to 

 those which take place in different rocky strata, not only where they approxi- 

 mate to granite, but where they are in contact with masses or veins of the 

 trap rocks. The production of chert and jasper is easily accounted for, by 



