162 Scienti/ic Proceedings, Royal Dublin Society. 



egg. If a dyke is a through one, large crystals of aragonite are 

 common on the roof, the pisolitic ore being also much altered 

 (baked), having in it numerous joints filled with carbonate of 

 lime, and under such circumstances, the ore comes out in blocks, 

 which if broken open, show very little of the pisolitic structure. 



The stop dykes usually have a decomposed or soft head, which 

 may extend for two feet in depth, sometimes however, they are 

 quite firm and hard at the top, and are separated from the roof 

 by a small parting of clay only. In GlenarifF mines, we have 

 one dyke which comes up and turns over, or " splashes " against 

 the roof, proving it to be newer (see Plate XIX.) The through 

 dykes vary from six to twelve feet thick, and the stop dykes 

 from two to six feet. 



On the surface of the ground some of the through dykes can 

 be traced to their termination, that is to the sheet of dolerite 

 into which they join ; the dyke No. 6,* in longitudinal section can 

 be seen to pass through the roof and overlying dolerites until it 

 joins into the uppermost sheet. The pisolitic ore where in con- 

 tact with this dyke is burnt into a hard mass, in which the 

 pisolites can be distinctly seen, though they cleave across with 

 the stone of ore when broken open. The dyke No. 7, which is 

 in a down throw of three feet to the west, and though much thicker 

 than No. 6, cannot be seen on the surface of the ground, only 

 bakes the ore slightly and probably belongs to the flow which 

 formed the roof or the one immediately above it. To the west 

 of this dyke there is scarcely any pisolitic ore, but immediately 

 to the east of it there is a magnificent seam probably the best in 

 Glenariff. 



No. 9 is a " stop dyke " which varies from two inches to two 

 feet thick. It is very irregular in its course, and in places 

 appears to have come up with considerable force, and splashed or 

 turned over against the roof as shown in the section ; but other 

 parts of it do not come up to the roof. Usually, however, the 

 stop dykes have a soft or decomposed head, the one shown No. 10 

 is six feet thick, and in places is decomposed for three feet in depth ; 

 to the west of this dyke there is scarcely any pisolitic ore ; but 

 immediately to the east of it there are two good seams (see Plate 

 XIX.), separated by a few inches of steatitic clay, the top seam is 



* This dyke was shown on a map and longitudinal section of the iron ore measures, 

 which is not published with this paper. 



