THE PROTEROZOIC ERA. 205 



on by the more refined methods of the present time, an unconformity 

 has generally been found. The unconformity is evident wherever the 

 overlying system is not highly metamorphosed; but where the strata 

 have been much folded and subjected to the changes which profound 

 dynamic action always brings in its train, they have sometimes be- 

 come so thoroughly crystalline and so complex in structure that they 

 are not easily differentiated from the Archean in the limited exposures 

 which are open to observation. The fact that the Archean was affected 

 by the same dynamic action which metamorphosed the Proterozoic 

 rocks tends to give the two systems uniformity of structure where 

 both have been profoundly altered. 



The Adirondack region. 1 — Rocks believed to be of Proterozoic age make up 

 the larger part of the Adirondack mountain mass. These rocks belong to two 

 groups: (1) The central mass of tri3 mountains, made up of a complex of igne- 

 ous rocks which were intruded into (2) a series of pre-Cambrian meta-sedi- 

 mentary rocks. The igneous rocks sometimes contain bodies of clastic rock. 

 The igneous rocks are of various types (granite, syenite, anorthosite), and are 

 massive in some places and much foliated in others. The clastic series into which 

 the igneous rocks were intruded surround and lap up about the base of the igne- 

 ous mass. They include crystalline limestone, and quartzose, sillimanitic and 

 graphitic gneisses, together with some gneisses believed to be meta-igneous. 

 The limestone (marble) associated with the gneisses is often massive, a structure 

 which suggests either that it was completely crystallized under static conditions 

 subsequent to the dynamic action which effected the metamorphism of the other 

 sorts of sedimentary rock, 2 or that it behaved very differently from the other 

 sorts of rock when metamorphism was in progress. The metamorphism of the 

 clastic series was probably largely effected at the time of the intrusion of the 

 igneous rocks. The meta-sedimentary rocks are thought to be the equivalent 

 of the Grenville series of Ontario. The floor on which these sedimentary rocks 

 were laid down has not been discovered. 



Isolated areas in the Mississippi basin. — Proterozoic rocks come to the sur- 

 face in isolated areas at several points in the basin of the Mississippi east of the 

 Rocky mountains (Fig. 38). There are some small areas of Animikean (?) quartz- 

 ite in south-central Wisconsin (the Baraboo quartzite), 3 and in the southeastern 

 corner of South Dakota 4 and adjacent parts of Minnesota and Iowa (the Sioux 

 quartzite) (Fig. 80). In the former place the quartzite has no association with 



1 Cushing, 54th Ann. Rept. N. Y. State Museum, Vol. I, pp. 258-282; and Kemp, 

 13th Ann. Rept. N. Y. State Geologist, 1893, pp. 444, 445. 



2 Van Hise, Principles of Pre-Cambrian Geology, 16th Ann. Rept., U. S. Geol. 

 Surv., p. 771. 



3 Irving, Geol. of Wis., Vol. II, pp. 504-519. This may be the Middle Huronian 

 of p. 161. Weidman, Bull. XIII, Wis. Geol. Surv. 1904. 



4 Todd and Hall, Alexandria (S. D.) folio, U. S. Geol. Surv. 



