124 DR GEIK1E ON THE HISTORY OF VOLCANIC ACTION 



out parallel with the upper and under surfaces of the bed. So marked is this flow- 

 structure that hand-specimens might readily be taken at the first glance for ancient 

 schistose limestone. " The felspathic ingredient (probably labradorite or anorthite) is 

 white, and its lath-shaped crystals have ranged themselves with their long axes parallel 

 to the line of flow. The olivine occurs in perfectly fresh grains, which in hand-specimens 

 have a delicate green tint. Under the microscope they appear colourless, and are pene- 

 trated by the felspar prisms in ophitic intergrowth. There is a small quantity of a pale 

 brownish augite, which not only occurs in wedge-shaped portions between the felspars, 

 but also as a narrow zone round the olivines." # Considerable differences are visible in 

 the development of the flow-structure, and with these there appear to be accompanying 

 variations in the microscopic structure. Dr Hatch to whom I submitted my specimens, 

 informs me that in one of them, where the flow-structure is so marked as to give a finely 

 schistose aspect to the rock, " there is a larger proportion of augite, some of which exhibits 

 a distinct diallagic striping ; the olivine grains show no ophitic structure, but are some- 

 times completely imbedded in the augite." To this remarkable flow-structure I shall 

 again refer in connection with the light it throws on the bedded character of the exterior 

 of the gabbro bosses. 



Between these different basic igneous rocks of the Inner Hebrides, as Professor Judd 

 has shown, there are many gradations according to the varying proportions of the chief 

 component minerals. Thus from the olivine-gabbros, by the diminution or disappearance 

 of the augite we get such rocks as troctolite ; where the plagioclase diminishes or 

 vanishes, we have the different forms of picrite ; where the olivine is left out, we come to 

 compounds, like eucrite ; while by the lessening or disappearance of the felspar and augite, 

 we are led to ultra basic compounds, consisting in greatest part of olivine like lherzolite 

 and dunite. 



§ 2. Relations to the other Volcanic Rocks. 



Various opinions have been expressed regarding the connection between the 

 amorphous eruptive rocks of the hill-groups and the level basalt-sheets of the 

 plateaux. Jameson, though he landed at Rudh' an Dunain, in Skye, where this 

 connection can readily be found, does not seem to have made any attempt to 

 ascertain it. He noticed that the lower grounds were formed of basalt, and that 

 the mountains "appeared to be wholly composed of syenite and hornblende rock, 

 traversed by basalt veins." t Macculloch, in many passages of his Western Islands, 

 alludes to the subject as one which he knew would interest geologists, but about 

 which he felt that he could give no satisfactory information, and with characteristic 

 verbiage he refers to the impossibility of determining boundaries, to the transition 

 from one rock into another, to the inaccessible nature of the ground, to the almost 

 insuperable obstacles that impede examination, to the distance from human habita- 



* MS. of Dr Hatch. 



t Mineraloyical Travels (1813), vol. ii. p. 72. 



