Time-table for North America. 21 



or the other division. For such areas the general name of 

 Laurentian will continue to apply, and may include rocks of 

 two widely different ages. Thus the field geologist is not 

 faced by the difficulty of indicating the age and relations of 

 the basement rock of the Canadian Shield before such are 

 known in his locality. 



The presence of vast batholithic invasions is not now 

 regarded as so distinctively a process related to earth origin as 

 it was formerly, though in the Laurentian it does seem to have 

 occurred on a grander scale than in any later era and in that 

 respect is doubtless related to the earlier stages of the earth. 

 The metamorphic province of the Appalachians, for example, 

 has been intruded by granite gneisses in the Paleozoic, the 

 extent of their exposure being shown by the blotches of red 

 on the geologic map of North America published by the U. S. 

 Geological Survey in 1911. The areas of concealed granites 

 and gneisses are without doubt of far wider extension, connect- 

 ing in depth what are now seen as isolated areas. If erosion 

 were to plane as deeply in the metamorphic regions of the 

 Appalachians, it would doubtless reveal there a basement 

 complex of Paleozoic and older rocks comparable in character 

 to the Laurentian. The Cordilleran Province is also widely 

 underlain by igneous rocks, but these are mostly of post-Paleo- 

 zoic date. In the Sierras and Coast ranges they have become 

 broadly exposed by erosion, and a deeper planation would there 

 also widen and unify the exposures of igneous rock. 



The recognition of at least two great periods of batholithic 

 invasion in the pre-Cambrian raises the question whether there 

 may not be more, and whether the basal fundamental gneiss in 

 different parts of the world, as Yan Hise has previously noted, 

 may not have varying ages. With increase of knowledge this 

 seems more strongly a possibility and should serve as a caution 

 against hasty correlations of widely different regions. Lawson 

 puts the great Algoman igneous irruption and the following 

 Eparchean interval between the Huronian and Animikian. 

 Others regard the greater break as below the Huronian. If, 

 however, batholithic invasion should be found to occur widely 

 at this horizon, it might result in a division of the pre-Cam- 

 brian of the Canadian Shield into four, in place of the present 

 three divisions, as these in turn now tend to supplant the older 

 usage of a two-fold division into Archean and Algonkian. 



Molten rock accumulates in reservoirs deep in the crust and 

 the higher intrusions and extrusions are given off from these. 

 But while standing quiescent, the fluid acts like an unstable emul- 

 sion. On the one hand, the lime, iron, and magnesia tend to 

 segregate more or less together, retaining less than the average 

 per cent of silica. This dominance of metallic oxides gives basic 



