DESCRIPTIONS OF THE FORMATIONS. 13 



the region just named, in the South Mountain in Pennsylvania, south of the 

 Susquehanna, and along the eastern coasts of Massachusetts and New Brunswick. 

 The Huronian rocks are penetrated in many cases by eruptive rocks, both granites 

 and dolerites. A series of rocks, which the writer has referred to the Huronian, 

 appears in parts of the British Islands, notably in Donegal, Ireland, in Anglesea, 

 and in Caernarvonshire. The crystalline rocks which underlie unconformably the 

 Lower Cambrian strata in South Wales, and to which the name of Dimetian 

 has lately been given, seem, from the descriptions, to belong to the Huronian 

 terrane. The great series in the Alps, called by the Italians the greenstone group, 

 or pietn verdi, has both the lithological characters, and the geognostic relations, 

 of the Huronian ; and the similar crystalline schists found in California, in the 

 foot-hills of the Sierras, and in the Coast range, are probably to be referred to 

 the same horizon. The gold-bearing veins of California are found both in these 

 crystalline schists and in the eruptive granites. 



1 d. Montalban, This name was given, in 1872, to a great mass of 

 crystalline schists, which are lithologically and geognostically distinguished from 

 the Huronian, and are well displayed in the White Mountains (whence their name). 

 They occupy large areas in New England, and constitute the gneisses and mica- 

 schists of New York Island, of Philadelphia, Baltimore and Washington. A 

 similar group of rocks is found at the summit of the Huronian series, in northern 

 Michigan ; and from this, as well as from the facts observed on the Schuylkill, and 

 many other places, they are believed to be younger than the Huronian, although 

 some geologists have supposed them to be older. Similar rocks are traced south- 

 westward from the Potomac, throughout the Blue Ridge, of which they form, in 

 Virginia, North and South Carolina and Georgia, an important part, and are there 

 gold-bearing. 



The gneisses of this series are distinguished from those of the Laurentian by 

 being finer grained, and having white feldspar. They are, moreover, less firm, and 

 more tender, often containing silvery mica, and pass by insensible gradations into 

 the coarse mica-schists of the series, which are very unlike in aspect to the soft 

 unctuous mica-schists of the Huronian. Hornblende prevails in many parts of 

 the series, and the gneisses, by its predominance, pass into a bluish-black hornblende- 

 rock, often thin-bedded. Noticeable among the basic members of the terrane, is 

 the granular olivine or chrysolite-rock, which, often accompanied by enstatite, and 

 by serpentine, appears to be interstratified in the micaceous and hornblendlc 

 schists of the Montalban, hi North Carolina, and in Georgia. Crystalline limestones 

 are found in this terrane, often in considerable masses, and resemble somewhat, in 

 the presence of hornblende, apatite and graphite, the limestones of the Laurentian. 



The Montalban series exhibits beds and veins of iron-pyrites and copper-pyrites, 

 in many localities, but the oxidized iron-ores which abound in the preceding 

 series, are scarcely known in this. The fine-grained gneisses of the Montalban, are 

 commonly, known in New England by the name of granites, but the series is also 

 penetrated by great masses of true eruptive granite. The mica-schists of the se- 

 ries are remarkable for the abundance of crystallized garnet, staurolite, chiastolite 

 and cyanite which they contain ; these species, with the exception of the first, not 

 being, so far as known, found hi the Laurentian series. The endogenous granitic 

 veins, carrying muscovite, dichroite, spodumene, tourmaline, beryl, columbite, 

 tinstone, and apatite, in the Atlantic belt, are chiefly, if not wholly, found in the 

 Montalban series. T. STEERY HUNT. 



