ARCH^AN TIME. 



153 



xocks are in sheets. This grand fact is, then, evident, — that the 

 Archaean rocks are stratified, as much as the rocks of any later age. 



In the series in the region of Ottawa, there are three great limestone strata, separated 

 ty gneissoid rocks, in all not less than 3,500 feet in thickness. The upper of these 

 limestones is about 1,500 feet thick; but nearly half consists of intercalated layers of 

 gneiss, and the limestone of each stratum is often associated with, or passes into, rocks 

 consisting largely of pyroxene or hornblende; and these portions often abound in 

 minerals, the most common of which are graphite, orthoclase, mica, scapolite, wol- 

 lastonite, sphene, serpentine. 



The following section by Logan (real in its general truths, although partly ideal) 

 exhibits well the fact and condition of the stratification. It presents to view a stratum 



Fin-. 207. 



of (a) white granular or crystalline limestone, many times folded, and interstratified 

 with gneiss and quartz rock (b); and the limestone has been traced over the same 

 region (Grenville and adjacent country, Canada), in linear and curving bands corre- 

 sponding to a series of folds. 



The following figures represent iron-ore beds alternating with other strata. In 



Fig. 208. 



Fig. 209. 



Fig. 210. 



Fig. 208 (from the Michigan region, Foster and Whitney), the iron-ore, in extensive 

 beds (i, i), occurs between chlorite slate (a, a) and dioryte (i); and the iron-ore in i 

 is banded with jasper. In Figs. 209 and 210 (Essex County, N. Y., Emmons), the 

 iron-ore, in beds several yards wide, is associated 

 -with gneiss and quartz rock, and is_ interlaminated 

 with quartz, the whole dipping together in a com- 

 mon direction, like beds of sandstone, shale and 

 iron-ore, in many regions of sedimentary rocks. 

 At the Adirondack mines, in Essex County, N. Y., 

 one bed, according to Emmons, is 150 feet thick. 



In Fig. 211 (Penokie Eange, south of Lake 

 Superior, C. Whittlesey), h is hornblende rock and 

 slaty quartz; g, quartzyte, 30 feet thick; i, a bed of iron-ore, 25 to 50 feet thick. 



In the Missouri region, at Pilot Knob, — a hill 662 feet high above its base, — there 

 is a bed of hematite, 46 feet thick, overlaid by 140 feet of porphyry-conglomerate, and 

 underlaid by a red jaspery porphyry and other porphyritic rocks; and the ore-bed is 

 divided into two parts by a layer of slate ten inches to three feet thick. The pebbles 

 of the porphyry-conglomerate are cemented by iron-ore. The rocks of the region 

 also include granyte. (Pumpelly.) When the region was first visited, the surface of 

 the hill was covered mostly with huge blocks of the ore. 



The iron-ore, which is found so very abundantly in each of these regions, is partly 



