Vol. 57.] THE CRUSH-COIN^GLOMERATES OF ARGYLLSHIRE. 317 



and metamorphism. In this particular district, of which the crush- 

 conglomerate area forms a part, the igneous rock so exceeds the 

 sedimentary material that over a large tract of country it may be 

 said to play the role of groundmass. A zone of this rock i& 

 continuous for a breadth of 5 miles, among which discontinuous 

 strips of sediment are embedded, while fringing this big mass, the 

 sills of epidiorite, though large and numerous, form discontinuous 

 intrusions enclosed by sediments. 



Although portions of this large mass, which extends from Kilmichael 

 to Loch Awe, may be described as normal epidiorite, it 

 embraces rocks of such totally different type that this term would 

 be altogether misleading applied to such local variations. But as it 

 includes much typical epidiorite, and as there is very little doubt 

 of its being allied with other sills of like type in the district, it 

 finds its place naturally within that group. It shares in the 

 metamorphism of the sediments with which it is associated, and, 

 like those deposits, it varies in the amount of alteration that it has 

 undergone. The mass, which is sometimes vesicular, varies in com- 

 position from basic to intermediate. The dominant minerals are 

 hornblende and plagiocJase-felspar, with chlorite, epidote, calcite, 

 quartz and iron-ores as the more important accessories. There is 

 every gradation in structure, from a coarse gabbro-like type to the 

 finest schist. It is sometimes very coarse, often foliated, con- 

 sisting of hornblende and felspar, the hornblende occurring in platy 

 crystals up to | inch in length, and the felspar in elongated prisms, 

 while epidote is abundant. Zones of finer texture composed of horn- 

 blende and felspar also occur ; in these the rock is massive and 

 compact, and approaches a fine schistose diorite in character. 



The most prevalent type is a felspar-hornblende-schist 

 with porphyritic felspars scattered about in it. With this type bands 

 of very fine chlorite-schist are commonly seen, in which the 

 porphyritic felspars are equally abundant. Eapid alternations occur 

 of zones in which the amount of porphyritic felspar varies, in some 

 bands being altogether absent. Similarly, the groundmass of the 

 rock varies both in texture and in composition. These differences 

 of structure and composition are often so sharply pronounced that 

 banding is produced in the rock. In the varieties where chlorite 

 takes the place of hornblende the rock occurs as a green schist or 

 slate which, when the porphyritic crystals of felspar are absent, 

 cannot readily be distinguished in the field from a sediment. 



The porphyritic felspars occur in idiomorphic crystals, but they 

 are often broken across and their outlines sometimes rounded, 

 yet notwithstanding the highly-sheared matrix they have, on the 

 whole, suffered very little crushing. They frequently measure half 

 an inch, and rarely 2 inches in length. In some zones they are so 

 packed together to the exclusion of the groundmass that the latter 

 is insignificantly represented. Although portions show the horn- 

 blende as porphyritic, this type is not common. 



While as a whole the rock is fairly fresh, both in the coarse dioritic 

 varieties and in the fine chlorite-schists, considerable masses are 



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