508 D)\ T. Sferri/ Hunt— The Eozoic Rochs of N. America. 



frequent presence of schistose rocks, and of conglomerates, whicli" 

 contain fragments of tlie underlying gneisses. These characters, 

 which are common to the Huronian and the two succeeding groups, 

 led the earlier geologists of America to include them among transi- 

 tion rocks. The Hui'onian contains a considerable proportion of 

 epidote, hornblende and pyroxene, and is marked by varieties of 

 diabasic rocks often called gabbros, whicli are truly stratified, but are 

 not to be confounded with the norites of the Norian series, to which 

 the name of gabbro is also often given. The Huronian series more- 

 over includes imperfect gneisses, quartzites, dolomites, serpentines 

 and steatite, besides large amounts of chloritic, micaceovis and 

 argillaceous schists. Its thickness is estimated at about 18,000 feet, 

 and it is often found resting unconforraably upon the gneiss of the 

 Laurentian. The Huronian appears to be identical with much of 

 what has since been called Pebidian in the British Isles, and with 

 the true pietri verdi group of the Alps, there found in many parts 

 between the ancient gneisses below and a younger series of gneisses 

 and mica-schists. 



There is met with in North America a similar series to these, to 

 which in 1870 the writer gave the name of Montalban, for the 

 reason that it is found largely developed in the White Mountains 

 of New England. This series contains fine-grained white gneisses, 

 sometimes porphyritic, but distinct from the granitoid gneisses of 

 the Laurentian, and passing into granulites on the one hand, and 

 into very quartzose coarse-grained mica-schists on the other. It 

 also includes hornblendic gneisses and black hornblende schists, 

 together with serpentines, chrysolite rocks, dichroite-gneiss, and 

 crystalline limestones. The mica-schists of the series often contain 

 garnets, andalusite, fibrolite, cyanite, and staurolite, while in the 

 granitic veins which traverse the series are found tourmaline, beryl 

 and cassiterite. The total thickness of the Montalban is apparently 

 much greater than that assigned to the Huronian, upon which it 

 sometimes rests unconformably, or, as is often the case in the absence 

 of this, directly upon the Laurentian. 



We come in the next place to a series composed essentially of 

 quartzites, limestone, and micaceous and argillaceous schists. The 

 quartzites, occasionally conglomerate, are sometimes vitreous, some- 

 times granular and often micaceous, passing into mica schists very 

 distinct from those of the Montalban. The mica is often damourite 

 or sericite, and gives rise to unctuous glossy schists passing into 

 argillites which sometimes contain a felspathic admixture. The 

 limestones of this series, often magnesian, are crystalline, and include 

 statuary marbles and cipolinos. We find in the schists, which are 

 intercalated alike among the quartzites and the limestones of this 

 series, masses of serpentine and of ophicalcite, and occasionally of 

 chloritic and hornblendic minerals, as well as siderite, magnetite, 

 and hematite, the iron oxides being often mingled with the quartzites. 

 These last are sometimes flexible and elastic, and the whole series 

 much resembles the Itacolumitic group of Brazil. It has a thickness 

 in different parts of North America of from 4,000 to 10,000 feet, 

 and is found lying unconformably alike upon the Laurentian, the 



