ROCKS OP THE NORTHERN HIGHLANDS. 387 



connexion between the limestones of the two localities can be 

 proved. 



It is first of all necessary to examine the chemical characters of 

 these rocks. There are two well-marked varieties — a cream-coloured 

 dolomite affected but slightly by hydrochloric acid, and a white or 

 grey marble effervescing freely under the test. The crystallization 

 of the marble is usually coarser than that of the dolomite. The 

 former type predominates in Dr. Heddle's lower series, and the dolo- 

 mite in the upper. It is, however, important to notice that neither 

 variety is confined to either subdivision. Dolomite of precisely the 

 same type as that which prevails in the upper zone is interstratified 

 with the marbles of the lower ; and near Kinloch Ailsh I found 

 abundance of the grey variety amidst the beds of dolomite. I have 

 also detected the crystalline dolomite further west in association with 

 the well-known marbles of Ledbeg. I do not, therefore, see any 

 reason for separating the Loch Ailsh limestones into two distinct 

 groups, though it is convenient to record that dolomite prevails in 

 the upper part and marble in the lower. 



I concede, contrary to the view of Nicol, that this limestone, at its 

 upper margin, is clearly intercalated with the Eastern Gneiss. 



That this limestone is a member of the Assynt series appears to be 

 at once disproved by its chemical composition. The Assynt rock is 

 a dolomite, and everywhere a dolomite. If it be suggested that 

 metamorphism may have caused dedolomitization, I reply that marble 

 is in several localities clearly interbedded with dolomite, and that it 

 is incredible that the metamorphosing agent should have altered a 

 stratum and left the beds above and below it unaffected. I have 

 also a right to ask for proof that intrusive igneous rocks, the assumed 

 cause of the change, are capable of converting dolomite into calcic 

 carbonate. 



The lithology of the Loch Ailsh series is in other respects dis- 

 similar to that of the dolomite. In the latter, throughout its whole 

 length from Loch Broom to the North Sea, I have never found a 

 single quartzose bed ; but the former contains towards the top a 

 strong band of quartzite (No. 101, p. 418), which may be traced from 

 the high road to near Ben More Lodge, where it is associated with 

 calcareous schist (No. 105, p. 419), quite unlike any rock in the 

 Assynt group. This quartzite, as will be seen from Prof. Bonney's 

 note, is more highly altered than the ordinary quartzite of the 

 younger series (No. 100, p. 418). 



I have not been able to find in the Ledbeg marble, the presumed 

 equivalent of the Loch Ailsh limestone, any support of the view I am 

 controverting. I have studied numerous sections of this rock between 

 Loch Ailsh and the hill-slopes west of Ledbeg, a distance of five miles. 

 Everywhere it appears to lie in isolated fragments. North-east of 

 Luban-Cromah its bedding is nearly vertical, striking to the north- 

 west, parallel to the Hebridcan ridge of Scounan. About a mile 

 above Loyne, on the south bank of the Ledbeg river, marble, grey 

 and yellow, in nearly horizontal beds, comes within a yard of un- 

 altered dolomite of the Assynt series, dipping west ; but the marble is 



