Mineralogy of Sky. 7 1 



excavations made for quarrying that marble. I have always found 

 the marble much indurated and very much coloured at the contact, be- 

 ing at the same time contaminated with silica and other earths; while 

 the syenite itself in the same places appears rotten. Although in a 

 general sense the contact of these two rocks may be said to be visible 

 in various parts of this tract, some accident or other always occurs 

 at the point of meeting, to prevent the real contact from being seen. 

 If it is of small extent it is overgrown with soil and herbage ; if it is 

 of great dimensions, there is a chasm intervening which is filled with 

 fragments and rubbish. Thus the contact at Kilbride, which, would 

 otherwise prove instructive, is overwhelmed and invisible. I was 

 equally unable to find any place on the eastern side where its junction 

 with the upper of the secondary strata could be -observed, though it 

 unquestionably takes place near Broadford ; the soil is every where 

 a deep peat completely covering the rocky surface. It may pro- 

 bably be found on the shores between Broadford and Scalpa. I 

 know not that there is much to regret in not having seen more of 

 its connections with these strata, since, if, as I believe is undoubted, 

 it has the same relation to them as the trap has, no instruction fur- 

 ther than that which we already have can be derived from such 

 knowledge. It will be found indifferently interfering with every 

 one of the strata, and consequently no judgment of priority or pos- 

 teriority to any can be formed respecting it. 



The basis of the syenite is a substance which having been gene- 

 rally received as a felspar, I shall describe as such, although not con- 

 vinced of a sufficient identity in the composition of these two sub- 

 stances. In its softest state it may be considered as a clay stone, 

 since it offers no differences in character, while in a state of some- 

 what greater induration it becomes a clinkstone, and when more 



