476 ME. J. B. HILL ON PEOGEESSIVE METAMOEPHISM [Aug. 1 899, 



coast of Eilean-nan-Coinean. In this section calcareous quartzose 

 beds and argillaceous beds sharply alternate, and, although the 

 dips are at high angles, the beds on the whole are seen to be 

 nearly horizontal. It matters not whether we are dealing with 

 dips of foliation-planes or dips indicating the hade of limbs of folds : 

 as factors in estimating the thickness of a series or the general 

 stratigraphical position of the series as a whole, they must be 

 disregarded. 



Although, for the sake of convenience, I have classified these rocks 

 in two divisions — namely, the Ardrishaig Series and the Loch Awe 

 Series — it is important to bear in mind that they form 

 part of one great group. The Ardrishaig Series passes 

 continuously up into the Loch Awe Series without a 

 break. Wherever possible, the black-slate zone has been taken 

 as an arbitrary divisional line between the two series. This zone 

 makes its first appearance with the main limestone. It does not 

 always come on shar^^ly, but small divisional partings of the material 

 often make their appearance in ribbon-like bands near the top 

 of the Ardrishaig slates. In sections where the limestone is not 

 represented, the accession of graphitic material is sometimes so 

 gradual that it becomes a matter of great difficulty to decide where 

 the Ardrishaig slates end and the Loch Awe Series begins. 



lY. The Inteusive Igneous Eocks of the Aeea. 



Both the Loch Awe and Ardrishaig Series are pierced by innu- 

 merable sills of epidiorite, hornblende-schist, and chlorite- 

 schist. They occur in such abundance that in a great part of the 

 district they occupy at least half of the entire area. Like the schists 

 in which they occur, these igneous intrusions vary in the amount of 

 alteration that they have undergone, not only among themselves, 

 but even in different parts of the same mass. Where the sediments 

 have undergone least alteration we find these igneous masses in 

 their most unaltered state ; as the sediments become more altered, so 

 do these igneous masses. They vary in composition from basic to 

 intermediate rocks. Their dominant minerals are hornblende and 

 felspar, with chlorite, epidote, calcite, and iron-ores as accessories. 

 Biotite is rare in the unaltered rocks. 



Many of them in the field are modified diorites, others are altered 

 gabbros. These coarse varieties are seldom very foliated, except at 

 their outer edges. A big, highly foliated mass will often contain 

 zones that are not foliated at all, though there is usually a tendency 

 for the crystals to arrange themselves with their long axes parallel. 

 There is every gradation in these rocks from a coarse gabbro- 

 structure to the finest schists. The hornblende and felspar have 

 a habit of occurring as porphyritic crystals, as well as forming part 

 of the groundmass. As a rule, in rocks where the felspar is 

 markedly porphyritic the hornblende is not porphyritic. But this is 

 not always the case : porphyritic crystals of both hornblende and 



