No. 5.] HUNT — PRE-CAMBRIAN ROCKS. 261 



consisting of a lower group of hornblendic gneisses without lime- 

 stones, and an upper group of similar gneisses, distinguished bj 

 interstratified crystalline limestones. 



These rocks were found by Logan and by Murray to be over- 

 laid, both on Lake Superior and in the valley of the upper 

 Ottawa, by a scries consisting of chloritic and epidotic schists, 

 with bedded greenstones, and with conglomerates holding pebbles 

 derived from the ancient gneiss below. The same overlying 

 series had, as early as 1824, been described by Bigsby on Lake 

 Superior, and by him distinguished from the Primary and classed 

 with Transition rocks. 



Labradoritic and hypersthenic rocks like those previously de- 

 scribed by Emmons in the Primary region of northern New 

 York, were, in 1853 and 1854, discovered and carefully studied 

 in the Laurentide hills to the north of Montreal, when they were 

 described as being gneissoid in structure, and as interstratified 

 with true gneisses and with crystalline limestones. In 1854, the 

 writer, in concert with Logan, proposed for the ancient crystal- 

 line rocks of the Laurentide Mountains, includinsr the lower and 

 upper gneissic groups already mentioned, and the succeeding 

 labradoritic rocks (but excluding the chloritic and greenstone 

 series), the name of Laurentian. In an essay by the writer, in 

 1855, the oldest gneisses of Scotland and Scandinavia were, on 

 lithological and on stratigraphical grounds, referred to the Laur- 

 entian series, and at the same time the name of Huronian was 

 proposed for the chloritic and greenstone series, which had been 

 shown to overlie unconformably the Laurentian in Canada. 



Previous to this, in 1851, Foster and Whitney had described 

 the Laurentian and Huronian rocks of Lake Superior as consti- 

 tuting one Azoic system of Metamorphic rocks, with granites, 

 porphyries and iron-ores of igneous origin ; and in 1857, Whitney 

 attacked the two-fold division adopted by the Canadian geological 

 survey, maintaining that the stratified crystalline rocks of the 

 region belong to a single series, with a granitic nucleus. The 

 observations of Kimball in 1865, and the later studies of Credner, 

 of Brooks and Pumpelly, and of Irving, have, however, all con- 

 firmed the views of the Canadian survey as to the relations of 

 the Laurentian and Huronian in this region. 



The primary age of the Highlands of southern New York, and 

 their extension in what is called the South Mountain, as far as 

 .the Schuylkill, was now unquestioned, but the crystalline rocks 



