

 Partl§ 1] ARCHZAN. 637 
PART J.—ARcHZAAN. 
§1. General Characters, 
From underneath the most ancient fossiliferous stratified 
formations there rises to the surface in many parts of the globe a 
series of thoroughly crystalline schists and massive rocks. These 
fundamental formations have been regarded by some writers as 
portions of the primeval crust of the globe—traces of the surface that 
first congealed upon the molten nucleus. They are regarded by 
others as probably metamorphosed sediments which may belong to 
many different periods of geological history. Apart from the 
disputed question of their origin, they are everywhere acknowledged 
to include the oldest known rocks. Hence in so far as geological 
history is recorded in rock-formations they must be taken as its 
starting-point. In attempting to fix their relative date we first 
observe that they lie unconformably below succeeding formations. 
But this relation obviously goes only a short way in establishing 
their chronology. Nor are lithological characters much more 
valuable for the purpose. It must be conceded that these rocks may 
haye originated at many widely separated periods of early geological 
time, but that as yet no means have been devised of establishing any 
generally applicable tests of their relative antiquity. Hence for 
want of any satisfactory means of discrimination, these ancient 
crystalline masses must be, at least provisionally, classed under one 
common name. ‘They were formerly, and are still by some writers, 
called Primitive; but the term Archean, first proposed by Dana, 
has been generally adopted for them both in America and in 
Britain. 
Archean rocks everywhere present the same general characters. 
For the most part they consist of gneiss in many varieties, passing 
into various schists, among which occur subordinate bands of 
hornblendic and pyroxenic rocks, limestone, dolomite, serpentine, 
quartzite, graphite, hematite, magnetite, &c. The rocks have a 
general stratified structure, but the individual beds often present 
great irregularities of thickness, being specially prone to a lenticular 
development. Occasionally they dip continuously for some 
distance at angles of 40° or less; but more usually they are greatly 
plicated, and sometimes exhibit the most extraordinarily complex 
puckerings. The gneiss shades off into a non-foliated rock which 
occurs with it in alternating bands, but is in structure a true granite. 
Occasionally bands of this granite wander across the foliation of the 
eneiss. But they evidently belong to the period and processes of 
the gneiss formation, and cannot be classed as later intrusive 
eruptions. Everywhere the various bands of gneiss, and inter- 
stratified layers of schist or other crystalline rock are intimately 
united to each other by a minute felting together of their component 
erystals, 
