Vol. 57.] CRUSH-CONGLOMERATES OP ARGILLSHIRE. 315 



The limestone at the base of the series is extremely variable in 

 character, ranging, in the same exposure, from a coarse calcareous 

 grit to a dark calcareous slate, with a normal limestone as an inter- 

 mediate stage, while occasionally a band of quartzite may be seen 

 associated with it. In its normal condition it is generally crystalline 

 a!]d blue in colour. Crystals of nearly black calcite are sometimes 

 present, giving the rock a darker hue. In its gritty condition, 

 which is very common in this area, the limestone-matrix contains 

 numerous grains of blue quartz, and, felspars often red, which may 

 exceptionally reach an inch in length. The quartz-grains may fre- 

 quently be seen in the normal, fine-grained, compact limestone. 



Black slates are associated with the limestone, and by the 

 accession of calcareous matter often merge into it. The grits which 

 are at the top of the series sometimes shade imperceptibly into the 

 gritty limestones. These normal grits often contain zones so 

 coarse as to approach the condition of conglomerates, the pebbles of 

 quartz and felspar being as large as almonds, with which finer- 

 grained grits and grey quartzites are associated. 



We occasionally see sediments of a greenish hue due to the 

 accession of chloritic matter in their composition, but these greenish 

 grits and slates pass into, and are clearly associated with, those of 

 normal type. It is not uncommon to find pebbles of quartz and 

 felspar embedded in a green slate-matrix, and green calcareous beds 

 are sometimes associated with the limestones. 



The rocks are everywhere folded, the folds being of isoclinal type 

 and very steep, often even vertical. The bedding is, on the whole, 

 not far from the horizontal, with nearly vertical foliation. In fact 

 the members of the Loch Awe Series are here lying in a gentle 

 trough of the Ardrishaig Series which they succeed, and while such 

 indications as are afibrded by dip and foliation point to their being 

 steeply inclined, in reality we are dealing with a thin series lying 

 approximately horizontal, the difierent members of which are often 

 repeated by folding. These sediments have, on the whole, suffered 

 very little from those agencies of metamorphism which farther 

 eastward have resulted in the destruction of original structures and 

 the production of normal crystalline schists. The metamorphism 

 in this area has been of a comparatively mild type, and is not 

 always uniform in character. Even in the most altered sediments 

 the clastic nature of the rock is always apparent, for although the 

 finer-grained grits have been crushed and granulitized, the larger 

 grains have been merely flattened ; while in the least altered zones 

 they have practically escaped altogether — microscopic examination 

 revealing no granulitization, and the beds are little removed in 

 condition from normal Palaeozoic sediments. 



With these sediments is associated much igneous material 

 belonging to the epidiorite-group, which is commonly distri- 

 buted as sills in this part of the Western Highlands. As these 

 rocks play so important a part in the crush-conglomerates it will be 

 necessary to describe them a little more fully than the sediments, on 

 account of their complex character, both in regard to composition 



Q. J. G. S. No. 227. z 



