THE ARCHEOZOIC ERA. 143 



from the original crust under the peculiar conditions of a very hot, 

 acid atmosphere. The granites and gneisses came to be widely known 

 as the Laurentian formation, on the belief that they constituted a 

 distinctive basal series; but it is now known, in many instances at 

 least, that they are intrusions into the schist series. They are therefore 

 younger than the latter. The gneisses are now regarded, in the main, 

 as metamorphic rocks derived from granites by metamorphic action 

 which mashed the massive rock down into the foliated condition. 



In the formation of both the surface flows and the intrusions, the 

 ascending lavas must have occupied numerous fissures or conduits 

 connected with the interior, and hence there came to be numerous 

 dikes and other forms of intrusions, traversing such parts of the Archear 

 formation as had already been formed. It is also to be borne in mind 

 that all subsequent igneous action required passageways for the lavas 

 through the Archean, and that they also gave rise to traversing dikes 

 and intrusions of later date. These later intrusions, many of them 

 called Laurentian by Canadian geologists, are of course not properly 

 of Archean age, but they are not always separable, and their presence 

 adds to the complexity of the Archean as a whole. 



Similar formations not Archean. — To avoid misapprehension, it 

 is to be observed that granitic intrusions are not confined to the schist 

 series above described. There are granitic embossments associated 

 with younger systems of rocks ; but it appears from present knowledge 

 that such intrusions are much more local and incidental than in the 

 Archean system, where they are so dominant as to constitute a leading 

 and distinctive feature. It is also to be observed that there are, in 

 various regions, great series of gneisses and schists, with associated 

 massive crystalline rocks, which are not Archean at all, but meta- 

 morphosed portions of later formations. 



A special characteristic feature. — A not uncommon feature in the 

 American Archean is a series of numerous thin sheets of igneous rock 

 a few feet or a few yards apart, which penetrate the gneisses or schists 

 in directions more or less parallel to their foliation, giving rise to a 

 coarse interleaving of later igneous and earlier foliated rock. This 

 is so common as to be regarded by Van Hise as characteristic of the 

 Archean, since it is not known to prevail in any later formation. These 

 interleaved dike-sheets are commonly truncated at their junction with 

 the overlying Proterozoic formations, which shows that they were 



