Origin of the Crystalline Rocks. 471 



not unlike those of the Laurentian. This series, which abounds in 

 great beds of titanic iron-ore, has a volume which may exceed the 

 thickness of 10,000 feet assigned to it by Logan. The Laurentian is 

 in many parts overlaid by the Huronian series, which is charac- 

 terized by a great development of greenstones, generally horn- 

 blendic, with epidote, chlorite, steatite, serpentine, and soft hydrous 

 mica-schists, often called talcose, besides argillites, quartzites, and 

 limestones, generally magnesian. It abounds in metalliferous 

 deposits, including magnetic and specular iron-ores, chrome, and 

 sulphurets of copper, iron and nickel, and has assigned to it in 

 different regions a thickness of from ten to twenty thousand feet. 



In many parts of North America there exists a great development 

 of rocks characterized by the predominance of orthofelsite, or petro- 

 silex, often becoming a quartziferous porphyry. This, which is 

 apparently the helleflinta of Sweden, was regarded as eruptive until 

 in 1869 the author showed it to be a stratified series with some 

 associated quartzites and schists, and then included in the lower part 

 of the Huronian. Hitchcock, who has since studied these rocks in 

 New Hampshire, has called them Lower Huronian. From their 

 absence in many localities at the base of the typical Huronian, it is 

 conjectured that they may belong to a more ancient and distinct 

 series. The Montalban, or White Mountain series, is characterized 

 by micaceous gneisses, generally called granites, which pass into 

 quartzose and felspathic mica- schists, often abounding in garnets, 

 staurolite, fibrolite and cyanite. Great masses of dark green gneiss- 

 oid hornblendic rock, very distinct from the Huronian greenstones, 

 abound in the Montalban, which also includes beds of a very 

 peculiar olivine-rock, beside quartzites and crystalline limestones. 

 This series abounds in endogenous granitic veins, containing muscovite, 

 beryl, tourmaline, apatite, and oxide of tin. It probably equals the 

 Huronian in thickness, and is supposed to overlie it. The Taconian 

 series includes a great volume of characteristic mica-schists, often 

 quartzose, but seldom distinctly felspathic, and frequently con- 

 sisting in large part of damonite, or of pyrophyllite. Some of these, 

 like the schists of the Montalban, include garnet and chiastolite. 

 They are associated with quartzites, and with dolomites and lime- 

 stones, all of which are also frequently micaceous. Associated with 

 these are found serpentines and granular hornblendic rocks of a 

 peculiar type, very unlike those of the preceding groups and much 

 less crystalline. The quartzites are in large part detrital rocks. 

 This series, which yields the statuary marbles of North America, 

 has a thickness of about 5,000 feet, and is the Lower Taconic of 

 Emmons. It is found reposing alike on the Laurentian, Huronian, 

 and Montalban, and is overlaid, in apparent unconformity, by the 

 Upper Taconic, which is identical with the Quebec group of Logan. 

 This, which consists of many thousand feet of sandstones and 

 argillites, with some limestones, includes among its strata organic 

 forms belonging to various divisions of the Cambrian up to the 

 Arenig. The Taconian, although containing an undesci'ibed unguloid 

 shell, and a so-called Scolithus, is, by the author, considered pro- 



