L. A. Necker de Saussure Voyage en Ecusse. 177 



The grunstein wears thinner towards the northern extremity of 

 the hill, where it seems to terminate in the form of a wedge ; a 

 phenomenon which cannot be ascribed to the destructive action 

 of the elements, since the thinnest portion is now covered by 

 other strata which still exist in their natural position, and thus 

 have preserved this part from degradation. A hornblende of 

 a greenish-black, or bottle-green colour, constitutes two- 

 thirds of the grunstein rock ; the rest is a feldspar, which va- 

 ries from red to white. Small transparent plates, or milk- 

 white globules, of calcareous spar, are scattered throu^^h it. 



The grunstein, by long exposure to the weather, is decom- 

 posed, loses its lustre, assumes an earthy aspect, and a colour 

 of a livelier green. The angular and irregular masses of which 

 the rock is composed, lose their angles, and form balls com- 

 posed of very thin concentric lamina;, which are separated and 

 destroyed in succession. The upper surface of the rock is that 

 which has been most exposed to the action of the elements. We 

 find the grunstein has taken the precise appearance of a porous 

 lava. 



Let us now see what happens at the junction of these two 

 mineral masses. The principle of the superposition of rocks, 

 on which the whole of modern geognosy rests, seems to Mr, 

 Necker but one of the objects of this study. This principle 

 shews to a certain point the relative antiquity of the different 

 classes of rocks, but it goes no further. We must examine the 

 rocks in themselves, their structure, and their relations with 

 one another, before we can become acquainted with the agents 

 which have co-operated at their formation, and the particular 

 mode of this formation. Such, however distant at present, is 

 the true end of the vast study of geology. Now, asks he, 

 what can more promote its attainment, than to inquire if 

 any rock has, by its presence, influenced in any manner the 

 nature of the adjoining rocks, and introduced some modification 

 into their arrangement or composition ? It is rare to find the 

 different species of rocks in immediate contact; it is still rarer 

 to find such as have modified in any way by their presence, the 

 adjoining rocks ; a circumstance, perhaps, peculiar to those of 

 the trap formation. Dr. Hutton is the first geologist who exa- 

 mined minutely the phenomena which accompany the junction 

 of the trap rocks with those of the other class. The theory, 

 however, which he invented to explain these interesting facts, 

 appears, like all the other theories of the earth hitherto ima- 

 gined, liable to insurmountable objections of both a general 

 and particular nature. 



In Salisbury-crags, nothing can be more decided, or clearer, 

 than the line which separates the reddish-white sandstone from 

 the dark-green diabase or whin. Hand specimens are readily 

 procured, in which the two rocks occupy the opposite surfaces. 



Vol. Xlll. N 



