Dr. MacCulloch on the Geology of Glen Tilt. 275 



structure which occurs in the former. Where the marble is at the 

 greatest distance from the granite it differs little or nothing in hard- 

 ness or composition from ordinary specimens of this substance. But 

 wherever it approaches or comes into contact with the granite it be- 

 comes highly indurated, effervesces slowly with acids, and gives on 

 analysis a large portion of siliceous matter. In other respects there is 

 no obvious change at the planes of contact, and the union between the 

 granite and limestone is so slight that the action of the water sepa- 

 rates them. The union of the limestone with the schist is much 

 moi*e intimate. Wherever the granite traverses both these sub- 

 stances together, which is generally the case where the schist is pre- 

 sent, such a confusion takes place, that the boundary of neither can 

 be defined, a troubled mixture and then a complete gradation between 

 these two rocks being the result.* 



On the right bank of the river above this rock a bed may 

 be seen, consisting of a brownish hornstone, or rather a compact 

 felspar, with a character somewhat porphyritic. It is one of that 

 great variety of rocks associated under the general name of porphyry 

 of which frequent examples occur in the course of the Tilt on 

 both sides of the valley ; more than I have thought it necessary to 

 mention. These are sometimes considered as veins, and sometimes 

 as beds ; I believe that the former disposition is by far the most 

 common, but their real nature is often so difficult to ascertain that I 

 have been obliged to abandon the point in despair. The present one 

 appears to be a bed. 



I have purposely avoided noticing the rocks on both sides of the 

 liver which lie beyond its immediate bed, as they offer, on the 

 right side at least, a succession so mixed and so confused, that any 

 view of them which can be obtained in following the stream is nearly 



* Vide Plate 19.. 



2m 2 



