No. 5.] HUNT — PRE-CAMBRIAN ROCKS. 265 



siliceous and porphyritic slates, which form the chief part of 

 the Lower Coldbrook or inferior division of the Huronian series 

 in New Brunswick. The petrosilexes of Massachusetts were, 

 after careful examination by the writer, described by him in 

 1870, and in 1871, as indigenous stratified rocks forming a part 

 of the Huronian series. He subsequently, in 1871, studied the 

 similar rocks in south-eastern Missouri, and, in 1872, on the 

 north shore of Lake Superior, but was unable to find them in 

 the Green Mountain belt, or in its southward continuation, until, 

 in 1875, he detected them occupying a considerable area in the 

 South Mountain range in southern Pennsylvania. The stratified 

 petrosilex rocks of all these regions were described in a com- 

 munication to this Association, in 1876, as apparently corres- 

 ponding to the hdlleflinta rocks of Sweden, and, having in view 

 their stratigraphical position both in that country and in New 

 Brunswick, they were then "provisionally referred " "to a position 

 near the base of the Huronian series." Their absence in the 

 Huronian belt in western New England, and in the province of 

 Quebec, as well as at several observed points of contact between 

 Laurentian and the well-defined Huronian in the nothwest, led 

 to the suspicion that these halleflintas might belong to an inter- 

 mediate series. 



C. H. Hitchcock has pointed out that the characteristic Hu- 

 ronian rocks do not form the higher parts of the Green Mountain 

 range in Vermont, which he conceives to belong to an older 

 gneissic series, a conclusion which the writer regards as prema- 

 ture. Hitchcock, however, in his final report on the geology of 

 New Hampshire, in 1877, adopts the name of Huronian for the 

 crystalline rocks of the Altered Quebec group of Logan, which 

 makes up the chief part of the Green Mountain range in Quebec, 

 is largely developed along it in Vermont, and appears in a paral- 

 lel range farther east, which extends southward into New Hamp- 

 shire. In his tabular view of the geognostical groups in this 

 State, Hitchcock assigns to these rocks a thickness of over 12,000 

 feet, with the name of Upper Huronian ; while he designates as 

 Lower Huronian the petrosilex series of eastern Massachusetts, 

 already noticed, where these rocks are of great, though undeter- 

 mined, thickness. The similar petrosilex or halleflinta rocks in 

 Wisconsin, where they have lately been described by Irving as 

 Huronian, have according to this observer, a thickness, in a 

 single section, of 3,200 feet. They here sometimes become 



