120 Proceedings of the Royal Irisih Academi/. 



The hornblende-rock often merges into a nodular variety, or, 

 as previously stated, it may graduate into hornblendyte. (See figs. 

 L, M, and ^, PI. 10). 



Often between it and the hornblendyte there is no rigid boundary, 

 their mineral composition apparently being identical, on which account 

 the derivate rock would seem to have been formed from fine particles 

 abraded off the mass, or from the outer portion having been pul- 

 verised, and disintegrated fi'om its contact with the water into which 

 the rocks were protruded, while subsequently they were re-aiTanged 

 or stratified.^' 



Nodidar, or splieroidcd hornhlende-roch, is made up of blocks, or 

 iiTegular spheroids, fi'om the size of a man's fist to five or six feet in 

 diameter, irregularly heaped together, the interstices being filled with 

 a schistose substance, which is sometimes more or less curved round 

 the blocks, or with a felsitic-schistose rock, or even with quartzitic 

 stuff, or a mixture of their substances. A very peculiar variety of 

 this rock, only observed in one or two places, had a felsitic-schistose 

 matrix, very like felspathic-dioryte, except that it is foliated, in 

 which the blocks of hornblende-rock were enveloped. This rock merged 

 into the hornblende-rock, but not into the associated gneiss and 

 schist. 



Usually, as previously stated, the nodular-homblende-rock merges 

 into congiomcritic schists or gneiss {^metamorphosed agglomerate) so gra- 

 dually, that no boundary can be di'awn between the two kinds of rock ; 

 for which reason it would appear that this variety is not originally a 

 normal igneous rock, but rather represents the broken up jDortions of 

 the flows, rounded by abrasions against one another, or by weatheiTQg, 

 while the interstices were filled by the abraded and disintegrated por- 

 tions, sometimes combined with foreign substances. Many lava flows 

 have margins of loose blocks, that are thus described by Smyth : — 

 " Cooling and hardening on the surface, cracking, breaking up, and 

 falling forward in clinkery masses of rattling cinders and stones — such 

 must have been the mode of progression of these black streams, as 

 with many of the Yesuvian lavas under ocular inspection. "f And 

 if such had been the margin of the ancient flows in West Galway, 

 we could well imagine that now they graduate into a nodular 

 rock, not a normal igneous rock, or yet a typical tuff. Scrope,J how- 

 ever, and others, allude to the spheroidal and concretionary structure 

 of some basalts, and some of these rocks might possibly have so 

 originated. 



The nodular hornblende-rocks graduate into the previously de- 

 scribed conglomcritic-schist and eneiss. 



* Forbes has microscopically examined one of these schists, and proved that it 

 belonged to the derivate rocks. Microscope in Geology, pp. 13 and 15. 

 t " TenerifPe," by C. Piazzi Smyth, F. R. S., p. 249. 

 X " Yolcanos," p. Ill, and pp. 184, &c. 



