261 JDr. C. Callaway — On ConqKirative Lithology. 



made up of six or eight similar varieties. In both groups the degree 

 of crystallization varies between the slightly altered and the 

 minutely crystalline ; but in Ulster the general aspect of the series 

 is rather more schistose. If the L. Foyle Series is not Pebidian, 

 what is it ? Fossiliferous Caradoc strata occur on the margin of the 

 Ulster schistose area, but the former are mere shale and sandstone, as 

 unaltered as any English Ordovician rocks, and displaying no signs 

 of transition towards the hypocrystalline state. If the L. Foyle Series 

 is claimed as Palaeozoic, I have a right to demand proof of the 

 assertion. I should also like to inquire why a metamorphosed 

 Palaeozoic group should present an assemblage of rocks resembling 

 so closely the Pebidians of Leinster and Anglesey. 



The conclusions which I have been advocating are strongly 

 supported by a comparison with some distant areas. Of these, 

 space will not permit me to give more than one example, which I 

 select for the clearness of the evidence. Mr. J. E. Marr, in his 

 paper "On the Pre-Devonian Eocks of Bohemia,"^ states that beneath 

 the Lower Cambrian strata he found two well-marked groups. The 

 older was gneissic. It furnished pebbles to the younger, which is 

 described as having been "subjected to but slight metamorphism, and 

 that only in places," and consisting of " green schists, grits, ashes, 

 and breccias, interstratified with variously coloured hornstones." 

 These rocks in their turn supply rounded fragments to the Lower 

 Cambrian. Mr. Marr refers the younger Pre-Cambrian series to the 

 Pebidian, the older to the Dimetian. This interesting case shows us 

 that in Pre-Cambrian times similar conditions extended over wide 

 ai-eas, and confirms our faith in the utility of comparative lithology, 

 when applied with due precautions. 



I have thus given in outline the reasons why, amongst the 

 Archaean rocks, lithological evidence possesses a value sui generis. 

 I hold that the principles which regulate its application to ordinary 

 sedimentary strata and to igneous rocks do not apply to the crystalline 

 schists. That Pre-Cambrian rocks should have been exposed to con- 

 ditions, mechanical, thermal, and chemical, which did not extend, or 

 extended but partially, into Palaeozoic times, is only what we should 

 expect. What those conditions were is one of the most important 

 problems presented by our science ; yet I cannot but think that the 

 question will hardly be ripe for settlement till we have ascertained 

 with certainty the true Archaean succession. In America, some 

 fundamental points, such as the relation of the Huronian to the 

 Montalban, are in hot dispute ; and though the light seems a little 

 clearer in our own area, we can do little more than see men as trees 

 walking. Some amongst us regard our Archaean studies as barren 

 and unpromising. It seems to me, on the other hand, that our investi- 

 gation is opening out most fruitful fields of labour. We are working 

 at the great question of the origin of the crystalline schists, and 

 striving to throw light upon some of the earlier chapters in the 

 earth's crust. At present, the light is dim. " Let it grow." 



^ Quart. Joum. Geol. Soc. November, 1880, p. 591. 



