60 Dr A. Murray uit the Injiuencc uf Hoiks 



accordioo- to genera or to natural orders, or upon any other 

 cognizable principle ? No one, however, so far as I know, will 

 venture to answer these questions, or to ofFer facts to connect 

 more than a few plants with particular rocks, the best, even if 

 fully admitted, not being comparatively more extensive than 

 the usual exceptions to every rule. On the contrary, it will be 

 found, that facts have an entirely opposite tendency. 



There are various ways in which facts might be adduced to 

 illustrate our subject. It appears, however, to be the most sa- 

 tisfactory plan, to select rocks differing from one another in 

 structure and composition ; but in other points, particularly in 

 elevation and latitude, not materially dissimilar, and to com- 

 pare their respective vegetations ; for it is clear, that if the ma- 

 jority of plants, which are common upon a certain description 

 of rocks, do also, coeteris paribus, grow and thrive upon a rock 

 entirely different, this circumstance would go far to set the 

 question at rest. I have therefore taken the native plants of a 

 primitive district in Aberdeenshire, thirty or forty miles in cir- 

 cumference, composed mainly of granite and gneiss, and com- 

 pared them with the vegetations of the secondary rocks around 

 Edinburgh, and also with that of the still newer formation in 

 the neighbourhood of Paris. These situations, in point of lati- 

 tude, differ to a certain extent, but, upon the whole, they are 

 for the present purpose very unexceptionable ; as these respec- 

 tive rocks are, in the view of the chemist, mineralogist, and geo- 

 logist, as different, I may say, as is possible. It is necessary to 

 exclude plants depending upon local peculiarities, such as al- 

 pine and maritime species ; since the fittest comparison is that 

 which respects species growing over rocks, which differ in no 

 material point unless their own nature. 



A certain number of the more important genera and natural 

 orders of botany have been selected, and the plants common 

 in the Aberdeenshire district, belonging to those orders and ge- 

 nera, have been traced through the tract around Edinburgh, 

 and throughout the environs of Paris. The following obser- 

 vations then have been made. 



All the plants belonging to the order Compositae, which can 

 with propriety be called common in the Aberdeenshire district 

 already alluded to, are these : SoncJms ai-vensis, S. oleraceus, 



