On the Tertiary Iron Ore Measures. 161 



large scales or flakes if not propped. Hence, the local name, as 

 usually, these scales are taken down, that is, cleared or "brushed" 

 off the roof. The brushing is evidently an altered or decomposed 

 dolerite, while the clay is even more altered. They exhibit 

 numerous striation marks, and are more or less laminar; the 

 direction of the stria, so well marked in the Glenariff mines, 

 would possibly suggest, that the flow of dolerite which formed 

 the roof, came from the N.E. 



The dolerite roof over the brushing, is quite firm and hard, 

 and requires very little, if any, timber to support it; it invariably 

 exhibits a concretionary structure (or mammillary appearance), 

 similar to that seen above ground, in some weathered beds of 

 dolerite. Here, as in other districts, some beds weather into 

 spheroidal forms, while others decompose in layers, more or less 

 parallel to the original bedding of the stone. The hard dolerite 

 roof of the iron ore measures usually has, protruding from it, 

 crystals of labradorite, augite, and aragonite, the latter mineral 

 is also found filling joints in the roof. 



Across the iron ore measures, are dykes, which often displace 

 them ; those in the Glenariff district, have general bearings of 

 N.5.E., are perpendicular, or nearly so, and vary from a few 

 inches to several fathoms in thickness. Often they have a more 

 or less columnar structure, the columns lying nearly horizontal, 

 or at right angles to the walls of the dykes. The horizontal 

 joints, as a rule, are regular, extending from wall to wall; while 

 the perpendicular joints are not as persistent or regular. From 

 dykes in which both systems are developed, good building stone 

 can be procured. These dykes in the Glenariff mines, may be 

 divided into two distinct classes ; first, those which stop at the 

 roof, or " stop dykes," and second, those which penetrate the roof, 

 or "through dykes" the latter invariably displace the ore 

 measures, and bake the pisolitic ore seam; while the "stop 

 dykes," neither displace or bake the ore seam. The dykes that 

 stop at the roof, have a parting of steatitic clay, separating them 

 from it, similar to that on the pisolitic ore. All the dykes have 

 a film of steatitic clay at their walls, while the various joints are 

 coated with carbonate of lime. When approaching a dyke, 

 cracks in the pavement are frequently filled with acicular crystals 

 of aragonite ; the pavement being much tougher, and contains 

 numerous spots of bauxite, some of them being as large as an 



