THE ARCHEOZOIC ERA 41 



logical variations are many. . . . The Archean as a whole is homo- 

 geneous in its heterogeneity." 1 



Briefly stated, the Archean system exhibits the following char- 

 acteristics: (1) So far as observed, it always shows a profound un- 

 conformity or erosion surface at its summit; (2) its lower limit or 

 base has never been determined, and is likely inaccessible; (3) its 

 thickness is very great, at least tens of thousands of feet, and 

 possibly many miles; (4) its rocks are always crystalline and usu- 

 ally highly metamorphosed and tilted or folded; (5) it comprises 

 a most heretogeneous group of rocks, often intimately associated, 

 such as lavas and tuffs; shales, sandstones, and limestones which 

 have been highly metamorphosed to schists and gneisses, quartz- 

 ites, and marbles; some beds of iron ore; and great volumes of 

 granite or granitic gneisses; (6) almost invariably igneous rocks 

 (granites or lavas) greatly predominate; (7) it never contains 

 distinct fossils, though certain evidences of life do exist; and (8) 

 so far as known it is universally present at or under the earth's 

 surface. 



The Archean has been more or less studied in various countries, 

 and the above named features always appear to characterize it. 

 Caution must be exercised, however, in assigning groups of rocks 

 in different regions to the Archean merely because they present 

 some or many of these characteristics. Many rocks formerly 

 classed with the Archean have been proved to be of later age. If 

 rocks with all the characteristics of Archean lie below definitely 

 determined (by fossils) Cambrian strata, and are separated from 

 the Cambrian by a great series of sedimentary or metamorphic 

 rocks (Proterozoic) , then we may be pretty certain that the rocks 

 belong to the Archean system. If crystalline rocks of Archean 

 appearance are directly overlaid by Cambrian strata, or by Meso- 

 zoic strata, the crystalline rocks in the first instance may be either 

 Archeozoic or Proterozoic, and in the second instance of any age 

 preceding the Mesozoic era. 



Subdivisions of the Archean System. — Wherever studied the 

 Archean appears to be separable into two pretty distinct groups 

 or classes of rocks, namely, (1) a volcanic and sedimentary series, 

 and (2) a plutonic series. 



The volcanic and sedimentary series is largely composed of 

 metamorphosed lava flows and volcanic tuffs; some massive igne- 

 1 Van Hise and Leith: U. S. Geol. Survey Bull., 360, p. 26. 



