4 THE JOURNAL OF GEOLOGY. 



ical record. They are certainly both pre -Cambrian, but they 

 must belong to widely separated eras, and must have been pro- 

 duced by entirely different processes. If it is proposed to regard 

 the gneisses as "Archaean," we must refuse to include the Torri- 

 donian strata in the same section of pre -Cambrian time. But so 

 much uncertainty exists as to the application of this term Archaean, 

 examples are so multiplying wherein what was supposed to be 

 the oldest and truly Archaean rock is found to be intrusive in 

 rocks that were taken to be of much younger date, and there are 

 such slender grounds for correlating the so-called Archaean 

 rocks of one country with those of another, that I prefer for the 

 present, at least, not to use the term at all. Let me very briefly 

 state some of the main characteristics of the two sharply con- 

 trasted rock -systems of the north-west of Scotland. 



The oldest gneiss of that region was originally called " Lew- 

 isian" by Murchison, from its large development in the Island of 

 Lewis, and I think it would be, for the present at least, an advan- 

 tage to retain this geographical appellation. At first this 

 "fundamental gneiss" was thought to be a comparatively simple 

 formation, and the general impression probably was that it should 

 be regarded as a metamorphic mass, produced mainly from the 

 alterations of very ancient stratified rocks. Its foliation -planes 

 were believed to be those of original deposit which by terrestrial 

 disturbance had been thrown into numerous plications and corru- 

 gated puckerings. But a detailed study of this primeval rock 

 has revealed in it a far more complicated structure. The sup- 

 posed bedding -planes have been ascertained to have nothing to 

 do with sedimentary stratification, and the gneiss has been 

 resolved into a complex series of eruptive rocks, varying from a 

 highly basic to an acid type, and manifestly belonging to differ- 

 ent times of extrusion. With the exception of one district, to 

 which I shall immediately refer, no part of the whole region yet 

 examined has revealed to the rigid scrutiny of m)^ colleagues of 

 the Geological Survey, any trace of rocks which can be 

 regarded as probably of other than igneous origin. It is true 

 that our researches have been hitherto confined to the mainland 



