400 CAKBONIFEROUS * GNEISS ' AT GUTTANNEN. 



folded between older crj-stalline roclvS ; and in the locality where 

 the sedimentary beds attain their greatest development, the lower 

 strata contain well-marked conglomerates which include quartz- 

 pebbles together with many rolled fragments of gneiss and crystal- 

 line foliated schists. 



Gen. McMahon remarked that the term ' gneiss' ought nowadays 

 to be restricted to crystalline rocks of metamorphic origin, and 

 not applied to foliated eruptive rocks, on the one hand, or to clastic 

 rocks of detrital origin, on the other, however much the latter might 

 simulate metamorphic rocks in appearance. The use of the term in 

 a purely mineralogical sense led to many misconceptions. It seemed 

 unscientific to give the same name to rocks of totally different origin. 

 A vague term might have its convenience as a cloak for ignorance, 

 but now that the geologist had the microscope to aid him a more 

 precise terminology seemed desirable. 



Mr. EuTLEY commented upon the possible ways in which gneissic 

 and gneissoid structures may be produced. 



Prof. Blake remarked that the careful description given by the 

 Author put us in possession of the facts, and all that was left was 

 the logic of the nomenclature. If the rock in question showed the 

 fragments of the rocks whence it had been derived, so did every 

 detrital rock, though in the latter they might be smaller. The only 

 difference was the extent of the excursion from the original crystal- 

 line character. Hence the only way to escape calling this rock a 

 gneiss was to confine that term to such as show no trace of their 

 previous clastic character, in which case many of the pre-Cambrian 

 ' gneisses ' would be excluded. 



Prof. Seelet also spoke. 



Prof. Bonnet, in reply, stated that the specimens with fossils were 

 not less gneiss-like than the others. The best imitation of gneiss 

 among the specimens which he had collected was from the block 

 with plants. He quite agreed with Prof. Judd as to the importance 

 of dynamo-metamorphism : it was a truth, but not, he thought, the 

 whole truth. He knew the schists with Trilobites to which Prof. 

 Blake had referred, and they were very different from ordinary crys- 

 talline schists. His whole argument — and this applied to the 

 President's remarks — was not that these fragmental rocks had gone 

 back to a crystalline condition, but only that they looked like it, and 

 so misled observers. He maintained that a terminology could not 

 be founded upon appearances, and that if the same name was to be 

 applied to a crystalline rock and a rock which could be shown only 

 to imitate a crystalline rock — in short, to be only a forgery — the 

 whole science of petrology would be thrown into hopeless confusion. 



