474 ME. J. B. HILL ON PROGRESSIVE METAMOEPHISM [Aug. 1 899, 



The Loch Awe Series where least altered consists of : — 



(a) Limestone ; 



(6) Black slate (sometimes graphitic) ; and 



(c) Grits and quartzites. 



The limestone is very variable in character, and is intimately 

 associated with the black slates into which it sometimes insensibly 

 passes. It is usually crystalline, and in its most normal condition 

 is blue. Dark crystals of calcite are often very numerous, giving the 

 limestone a dark hue. Pebbles of blue quartz and pink felspar are 

 very commonly scattered through it ; they are sometimes so abundant 

 that the rock passes into a grit, with a calcareous matrix. These 

 pebbles of quartz and felspar attain sometimes a length of 1 inch, 

 but this is exceptional. The limestone occasionally contains pebbles 

 up to 6 inches in length, of limestone or slate. At times the lime- 

 stone is exceedingly fine-grained and compact, but in these fine- 

 grained compact beds pebbles of quartz may often be seen. 



The black slates are nearly always found in association with the 

 limestone ; they may occur above, or below, or in the limestone, and 

 sometimes, by becoming gradually more calcareous, pass into the 

 limestone, so that no line can be drawn where the limestone ends 

 and the slates begin. They have in many cases been quarried for 

 roofing-slates, and are occasionally graphitic, sufficiently so to soil 

 the fingers. 



The grits immediately succeed the black slates. They are com- 

 posed of pebbles of quartz and felspar, a large proportion of the 

 former being of the clear blue variety, while some of the felspar- 

 pebbles are pink, others being dull white. As a rule, the quartz- 

 pebbles preponderate largely over the felspar. Coarser and finer 

 pebbly beds rapidly alternate, and there are occasionally zones so 

 coarse in character as almost to deserve the name of conglomerates, 

 grit-zones of 2 or y up to 12 yards wide being composed of 

 pebbles of the size of almonds. The grits contain pockets of black 

 slate up to a foot or more in length ; the pockets are flattened, 

 and are seldom more than an inch thick. Partings of black slate 

 are also common in the grit-bands. These pockets and black-slate 

 partings are identical in character with the black slates already 

 described. The intercalations and pockets of argillaceous material 

 are characteristic of the grits in the Loch Awe Series. 



Finer-grained grits and grey quartzites are associated with 

 the coarser grits, these quartzites and fine grits containing the same 

 pockets and intercalations of black slate as those already described. 



Some of the grits are greenish, on account of the presence of 

 minute scales of chlorite. This chloritic matter, which has given the 

 prevailing green colour to the Ardrishaig phyllites, may sometimes 

 pervade the slates of the Loch Awe Series, and where free from 

 carbonaceous material has induced a green colour on them, instead 

 of the prevailing black and blue tint. The grits, however, may be 

 greenish, while the slate-partings are dark blue, or even black. 



In their least altered state, the condition of the Loch Awe grits 



