ANNIVERSARY ADDRESS OF THE TRESIDENT. 69 



movements in the original unconsolidated igneous masses, not yet 

 wholly effaced by later mechanical stresses. 



While there is little room for difference of opinion as to the deri- 

 vation of the material of the fundamental gneiss, a wide region of 

 mere speculation opens out when we try to picture the conditions 

 under which the material was erupted. Some geologists have boldly 

 advanced the doctrine that this gneiss represents the earliest crust 

 that consolidated upon the surface of the globe. But it presents no 

 points of resemblance to the ordinary aspect of superficial volcanic 

 ejections. On the contrary, the thoroughly crystalline conditions 

 even of those portions which seem most nearly to represent the 

 original structure of the mass, the absence of anything like scoriae 

 or fragmental bands of any kind, and the resemblances which may 

 be traced between parts of the gneiss and eruptive bosses of igneous 

 rock compel us to seek the nearest analogies to the original gneiss in 

 deep-seated masses of eruptive material. It is difficult to conceive 

 that any rocks approaching in character to the gabbros, picrites, 

 granulites, and other coarsely-crystalline portions of the old gneiss 

 could have consolidated at or near the surface. 



When the larger area of gneiss forming the chain of the Outer 

 Hebrides is studied, we may obtain additional information regarding 

 the probable original structure of the gneiss. In particular, we may 

 look for some unfoliated cores of a more acid character, and perhaps 

 for evidence which will show that from the igneous magma out of 

 which the gneiss came both acid and basic layers were segregated. 

 We may even entertain a faint hope that some trace may be dis- 

 covered of superficial or truly volcanic products connected with the 

 bosses of obviously eruptive nature. But up to the present time 

 no indication of any such superficial accompaniments has been 

 detected. If the bosses represent the deeper parts of columns of 

 molten rock that flowed out at the surface as lava with discharges 

 of fragmentary materials, all this superincumbent material had, at 

 least in the regions which have been studied in detail, disappeared 

 entirely before the deposition of the very oldest of our sedimentary 

 formations. 



So far, then, as the evidence now available allows a conclusion to 

 be drawn, the fundamental gneiss reveals to us a primeval group 

 of eruptive rocks presenting the strongest resemblance to some 

 which in later formations are connected, as underground con- 

 tinuations, with bedded lavas and tuffs that were erupted at the 

 surface. Though no direct proof has yet been obtained of true 



