GABBEOS ETC. IX SCOTLAND AND IKELAND. 



75 



paratively rare and eTen exceptional among the most deeplj'-seated 

 rocks (gabbros), they become more and more abundant as we pass to 

 less highly crystalline forms, till in the basalts they form the rule 

 rather than the exception. It may be fairly said that basalts are rare 

 in which evidence of «i^ hast two periods of consolidation is not repre- 

 sented among their constituents — the porphyritic crystals of felspar or 

 olivine or of both those minerals, and the glass filled with microliths. 



To sum up these results, we may say that among these basic 

 rocks the only type of rock ahsoliitely characteristic of intrusive 

 masses is the granitic; but ophitic varieties and varieties with 

 skeleton-crystals in their base abound in, though they are not con- 

 fined to, intrusive rocks: while rocks of granulitic structure and 

 those with short and rounded microlites in their ground-mass are 

 especially abundant among the lavas. 



The relations of the several rock-structures which we have been 

 describing to one another are illustrated in the following table. 



Vitreous. 



Basalt-glass (Tachylyte). 

 {Por^phyritic Tachylyte.'] 



With skeleton 



crystals in glassy 



base 



Micro-ophitic, 

 Ophitic. 



J 



Magma-Basalt. 

 \^Porphyritic Magma-basalt.'] 



Ordinary Basalt. 

 [Porphyritic Basalt.] 



DOLERITES. 



[Porphyritic Dolerites.] 



With granulai- 

 microliths in 

 glassy base. 



Micro-gran uliti c, 

 Granulitic. 



GrABBROS. 



[Porphyritic Gabbros.] 

 Granitic and Micrograuitic Structure. 



IX. GeIGIN of the SlRUCTrEES. 



The facts now passed in review cannot fail to suggest to every one 

 who carefully considers them an explanation of the origin of the 

 several structures we have been describing. 



The granitic structure exhibited by the gabbros is evidently the 

 result of the consolidation of a magma at a great depth from the 

 surface, where almost uniform temperatures are maintained during 

 enormous periods of time, and where the pressure is also very great. 

 Owing to the low conductivity of these rocks their rate of cooling- 

 down in such vast mountain-masses must be almost infinitely slow, 



