Dr. Mac Culloch on Quartz Rock. 459 



sicetch to which I refer, will doubtless also remind the spectator of 

 the view given in Dr. Fitton's paper, of the hills in the vicinity 

 of Dublin. The conical form of these hills appears to arise partly 

 from the rapidity of their decomposition, as far as regards their 

 mountainous bulk, and partly from the permanent nature of the 

 resulting fragments. To the same cause also is owing that particular 

 and arid appearance of sterility, which is so characteristic of this 

 class of mountains, an appearance which those who have seen Jura 

 will readily recognize, combined with a difficulty in ascending them, 

 which the geologist who has laboured in the attempt will not easily 

 forget. The peculiar whiteness which the surfaces of the fragments 

 show is however adventitious, as the fresh rock exhibits a variety of 

 grey, yellow, and brown tints, which long exposure to the atmosphere 

 will ultimately bleach. A narrower examination of unaltered spe- 

 cimens would possibly have prevented mineralogists from ever 

 applying to this rock the improper designation of granular quartz, 

 how much soever the weathered surfaces may resemble this substance. 

 On a nearer inspection, these mountains appear to consist of a 

 stratified rock, or to be formed of various beds of grit, which, where 

 the declivities are so steep as not to admit the lodgment of fragments, 

 become easily visible. In my remarks on Jura, I have described a 

 distinct and continuous bearing of the strata, but I was unable by 

 the aid of a spying-glass to perceive at any of the various points 

 from which I viewed the mountains of this coast, a similar tendency 

 through any considerable space. I should conceive their tendencies 

 to be exceedingly various, and that no general system of inclination 

 or direction predominated among them. But of this I must needs 

 speak with much diffidence, as I had no opportunity of ascending 

 any of the summits, from whence alone an accurate observation of 

 this nature could be made. Those who have been occupied in 



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