AECH^AN TIME. 447 



Algonkian formation. — The Algonkian formation (Aguotozoic of Irving) is made by 

 its describers to include the Huronian of Logan, nortli and south of the lakes, and some of 

 the so-called Huronian in other regions. Its rocks (1) comprise the thinner schists, semi- 

 crystalline slates, quartzytes, and uncrystalline f ragmental and shaly rocks ; and (2) they 

 are of pre-Cambrian age. The supplanting of the older name, Huronian, by the newer 

 is not sustained by any rules of nomenclature. It has been given a wider range by includ- 

 ing under it the Keweenaw copper-bearing sandstone formation, which lies unconformably 

 on the Huronian, and this change of limit was one reason for the change of name. 



T. B. Brooks first recognized the " Keweenawian " as a distinct system of rocks 

 (1876) ; Irving called it Keweenawan. If Archsean instead of Paleozoic, it marks a 

 Keweenawian period in the long Huronian era. The Keweenaw formation is without 

 fossils, and hence is of uncertain age ; but its relations appear to be probably Paleozoic, 

 as explained beyond. 



Some of the localities of Algonkian observed by Walcott are the following: (1) the 

 tilted beds of quartzytes and siliceous slates at the base of the Wasatch series, lying con- 

 formably beneath the Lower Cambrian; and (2) strata beneath the Cambrian in the Eureka 

 District and elsewhere in Nevada, where there is the same conformability. The beds are 

 described as very thick and as affording no fossils ; but the conformability to the Cambrian 

 suggests the query whether the beds are not lowest Cambrian. (3) At the base of the walls 

 in Grand Canon of the Colorado, lying unconformably beneath Upper Cambrian beds, up- 

 turned beds of sandstone, shale, and limestone, named by G. K. Gilbert, the Tonto group. 

 The presence of fossils in some of the Tonto beds (including remains of a Stromatoporid, 

 a Trilobite, and a Hyolithes, and a Discina-like shell) shows that part, at least, of the Tonto 

 group is not Algonkian, and renders it probable that all is Paleozoic. (4) In central Texas, 

 Llano County, beneath Upper Cambrian strata and over the Archsean, a formation which 

 is called the Llano group. (5) Part of the Huronian of southeastern Newfoundland, 

 described by Murray, which Walcott states is unconformable to the overlying Olenellus 

 beds. (6) Below the Potsdam series in the Adirondacks. These are some of the local- 

 ities of the so-called Algonkian formation. 



The facts respecting the Algonkian are reviewed in Van Hise's Report of 1892, men- 

 tioned above ; also briefly, on some localities, in Walcott's Correlation of the Cambrian, 

 U. S. Geol. Survey, Bulletin No. 81, 1891. 



Kinds of rocks. — The more cliaracteristic kinds of Archaean rocks are 

 coarse granites ; thick-bedded gneisses, especially hornblendic varieties, sye- 

 nytes, diorytes, and pyroxenic varieties of these rocks ; the granite-like rock 

 of the basalt type, called gabbro ; and each of these rocks under gneissic and 

 thin-schistose varieties. Zircon-syenyte is rather common. There are also 

 chrysolite rocks and chrysolitic varieties of some of the above kinds ; and 

 with them, serpentine rocks, the serpentine being a result of the alteration 

 of chrysolite or pyroxene and possibly of some other mineral containing 

 magnesia. 



Crystalline limestone (usually dolomyte or magnesian limestone) is 

 common in some regions ; and it often contains large crystals of apatite 

 (calcium phosphate) and the pale yellow mineral, chondrodite (a fluorine- 

 bearing magnesium silicate), supposed to be peculiar to the Archsean, 

 besides many other minerals. 



There are also in the Laurentian series, but less abundantly, horn- 

 blende schist, mica schist, hydromica (or sericite) schist, chlorite schist, 

 and quartzyte. 



