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mechanically formed rocks, produced chiefly by the agency of 

 water, and the igneous included all the plutonic and volcanic 

 rock produced by the agency of heat. Assuming that the 

 primary plutonic rock was granite, all the stratified rocks were 

 formed in layers over the granite, and the other plutonic and 

 volcanic works were formed below the granite. All igneous 

 rocks were once in a molten condition and liable to be forced 

 up through the stratified works. The granite zone gradually 

 cooled until it finally ceased as a volcanic rock, and then the 

 inner zone of basaltic rocks was projected into the stratified 

 rocks by the internal volcanic forces. The two zones of igneous 

 rocks gradually passed from one into the other as the thickness 

 of the cooling matter was increased from the surface to the 

 interior, so that this division into zones was quite arbitrary. 

 The division line between the two was determined by the 

 proportion of silica contained in the rock. Mr. Gray, by 

 diagrams, sections, and photographic slides, proceeded to 

 explain how the basaltic rocks were built up to their present 

 thickness through fissures or vents from below, which were now 

 known as dykes, of which the North of Ireland contained 

 numerous and very instructive examples. The result of the 

 continued operation of the forces which he enumerated in 

 geological and modern times was very well exhibited at the 

 Giant's Causeway. The extent, variety, and apparent strati- 

 fication of the basalt was very well shown on the exposed face 

 of Pleaskin, and was best seen from the spot known as Hamil- 

 ton's seat, a little to the east of Pleaskin and forming the 

 eastern terminus of the Causeway range of cliffs. That was the 

 spot from which the Rev. William Hamilton viewed and 

 studied the Causeway cliffs, and here he had collected the 

 information in his " Letters on the County Antrim." Having 

 pointed out the great variety of apparent stratified deposits 

 occurring in the face of the Pleaskin cliff, explaining that they 

 were of volcanic origin and of similar elements, that the 

 bedding indicated successive outbursts of volcanic matter, and 

 the lithological differences of the various beds were due to the 

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