84 The Eozoic Rocks of North America. 



gave the name of Huronian. It differs from those preceding 

 by the frequent presence of schistose rocks, and of conglomerates 

 which contain fragments of the underlying gneisses. These char- 

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

 groups, led some earlier geologists in America to include them 

 among Transition rocks. The Huronian contains a considerable 

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

 varieties of diabasic rocks often called gabbros, which are truly 

 stratified, but are not to be confounded with the norites of the 

 Norian series, to which frhe name of gabbro is also frequently 

 given. The Huronian series moreover includes imperfect gneisses, 

 quartzites, dolomite, serpentine and steatite, besides large amount 

 of chloritic, micaceous and argillaceous schists. Its thickness is esti- 

 mated at about 18,000 feet, and it is often found resting unconform- 

 ably 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 ofMontalban, 

 because it is found largely developed in the White Mountains 

 of New England. This series contains fine-grained white gneis- 

 ses, 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 serpentine, 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 Lau- 

 rentian. We come next to a series composed essentially of 

 quartzites, limestones, and micaceous and argillaceous schists. 

 The quartzites, occasionally conglomerate, are sometimes vitreous, 

 sometimes granulir and often micaceous, passing into mica-schists 



