2 THE JOURNAL OF GEOLOGY. 



It is obvious that in any effort to establish that a group of 

 rocks is older than the very base of the sedimentary fossiliferous 

 formations, we must somewhere find that group emerging from 

 under the bottom of these formations. Until lithological char- 

 acters are ascertained to be so distinctive and constant as to be 

 comparable to fossil evidence for purposes of stratigraphical 

 identification, we should not assume that detached areas of older 

 rocks rising amid Palaeozoic, Secondary or Tertiary formations 

 are pre-Cambrian. We should, if possible, begin at the bottom 

 of the Palaeozoic systems and work backward, tracing each 

 successive system or group as these rise from under each other, 

 until we arrive at what appears to be the oldest traceable within 

 the region of observation. It is clear that in the present 

 state of knowledge we have no satisfactory means of identifying 

 such successive systems in widely separated countries. All 

 that can be attempted in the meantime is to ascertain the special 

 types in each region, and to point out their general resemblances 

 or contrasts to those of other regions. It is better to avoid con- 

 fusion by refraining from applying the stratigraphical names 

 adopted for the oldest rocks of one region to those of another 

 geographically remote, though we may hope that eventually it 

 may be possible to work out the equivalence of these local 

 names. 



In the British Isles, by much the most important region for 

 the study of the oldest rocks is to be found in the north-west 

 Highlands of Scotland. The very basement strata of the Cam- 

 brian system are there traceable for a distance of more than lOO 

 miles, reposing with a strong unconformability upon all rocks of 

 older date. They consist of dolomitic shales with OloielliiSy 

 resting upon a thick group of quartzites, full of annelid tubes. 

 One of the most remarkable features of these ancient strata is 

 the persistence of their component bands or zones which, though 

 sometimes only a few feet thick, can be traced throughout the 

 whole tract of country just referred to. For the study of the 

 pre-Cambrian rocks this is an important point, for we can be 

 quite certain that even where fossil evidence locally fails, the 



