Dr. Mac Culloch on the Geology of Glen Tilt. 311 



llzed and conglomerated one. The island of Arran indeed pro- 

 duces some porphyry which appears to possess a structure analogous . 

 to this, inasmuch as grains of quartz apparently rounded, not 

 crystallized, are found imbedded in a common base with glassy and 

 earthy felspar. But thatstructure might admit of dispute. 



Such are the most remarkable of the independent facts which I 

 have noticed in this tract of country. Let us now, rejecting all 

 the minuter features which accompany this succession of rocks, 

 extend our views over a wider range, and attempt to investigate 

 their general bearing*. , 



The most continuous bed which has occurred in this view is the 

 limestone. We have seen that it is regularly bedded, without 

 any material disturbance, with an elevation somewhat varying 

 and consequently in an undulating plane, and that it is con- 

 tinuous over a very large tract of country. We have also seen 

 that its elevated edge is prolonged in a line which is straight for a 

 great distance, and which perhaps may prove to be so even 

 through a greater space, when an accurate survey of Scotland shall 

 have been produced. It is therefore the most regular rock of the 

 series now under examination, and may consequently be taken as 

 the point of comparison for the others and as that from which the 

 true relations of the rocks on each side of it must be investigated. 



It is indifferent to the object which I have at this moment in 

 view, whether the beds of limestone are considered as having been 

 originally deposited in their present position, or whether by subse- 

 quent changes they hav« been diverted from one more horizontal. 

 It will not be denied that they are deposited rocks. 



If we now trace upwards from this continuous bed of limestone 

 we shall find that it is followed by a large and continuous bed of 



* Vide Map, Plate 1. 



