ARCH^AN TIME. 457 



The crystalline rocks of St. Davids, in Wales, have been described by 

 Dr. Hicks as of three periods : (1) the Dimetian; (2) the Arvonian; and (3) 

 the Pebidian. Geikie concluded, after an examination of the region, that 

 the Dimetian rocks are intrusive granite; the Arvonian, " qiiartz-porphyries " 

 connected with the granite ; and that the Pebidian rocks are tufas and 

 diabases belonging to the lowest Cambrian. Dr. Hicks's view that the 

 St. Davids rocks are partly Archaean is favored by the presence in the 

 vicinity of fossiliferous Cambrian. It is now adopted by Geikie. 



In the Torridon district, northwestern Scotland, a thick formation of red- 

 dish and brownish sandstones, wholly uncrystalline in texture, but upturned 

 to a high angle, lies unconformably both upon Archaean gneisses and under- 

 neath strata of Lower or Olenellus Cambrian. The reported thickness is 

 4000 to 8000 feet. As they are uufossiliferous, it remains doubtful whether 

 the Torridon sandstone, or " Torridonian group," should be referred to the 

 later Archaean, or to the earliest Paleozoic. Murchison referred them to 

 the Cambrian. 



OBSERVATIONS ON THE ARCH^AN. 



1. Relations of the North American Archaean areas to the continent. — The 



position and form of the nucleal Archaean of the continent, and of the parallel 

 ranges on either side, reaching out to the oceans, prove that the continent 

 was not only outlined, but also marked off as regards its grander features in 

 Archaean time. This is established also by the great thickness of meta- 

 -morphic rocks ; for rocks of sedimentary or detrital origin are not made 

 except where there are emerged, or nearly emerged, rocks to be a source of 

 material ; and even a slight submergence makes the amount of decay, and 

 of detritus produced, small. Further, the existence of the continents, 

 emerged or at shallow depths, is evidence, as explained on page 380, that 

 the oceanic basin also was defined by the close of the Archaean, and had 

 nearly its present mean depth of 12,000 feet. 



The facts thus prove that the scheme of progress, even to minor details, 

 dates from the beginning. In the very inception of the continent, not only 

 was its general topography foreshadowed, but its main mountain chains 

 appear to have been begun, and its great intermediate basins to have been 

 defined. The evolution of the grand structure lines of the continent was 

 hence early commenced, and the system thus initiated was the system 

 to the end. Tracing out the development of the American continent, from 

 these Archaean beginnings, is one of the main purposes of geological history. 



2. Correlation of Archaean subdivisions. — Names of Archaean subdivisions 

 are multiplying over the world wherever Archaean rocks are studied. The 

 uncrystalline terranes are safely put at the top of the series in the particular 

 region where they occur; but, as already remarked, they may be the equiva- 

 lents of crystalline kinds in another more mountainous region. 



