1875.] 107 [Hunt. 



Very recently Prof. Pumpelly lias called attention to the evidence 

 that the similar decomposition of the stratified orthoclase-porphyries 

 of Eozoic age, with which are associated the iron ores of southeast- 

 ern Missouri, had already begun in early paleozoic time. 



It is known that in various parts in the northeast of the Atlantic 

 belt portions of decayed crystalline rocks are still found in situ, hav- 

 ing, from the accidents of position, been preserved from denudation. 

 I have to call the attention of the Society to a remarkable example 

 of this, which is seen at the Hocsac Tunnel, at North Adams, in this 

 State, where a good opportunity was afforded for studying the depth 

 of the decay. I have already given some account of it in my report 

 to the Corporators of the Hoosac Tunnel, in October, 1874, which 

 will be found published by the State, in House Document, No. 9, 

 January, 1875. 



The locality is at the western base of the Hoosac Mountain, the 

 crest of which here rises rapidly to a height of thirteen hundred feet 

 above the town of North Adams, which is itself seven hundred feet 

 above the sea. The mountain, a part of the north and south Hoosac 

 range, is traversed from east to west by a tunnel 25,081 feet in length, 

 the examination of which shows the rock to be chiefly micaceous 

 gneiss and mica-schist, including in its western half much hard fel- 

 spathic and quartzose rock, in part a granitoid gneiss. The strata 

 have a prevailing eastern dip, generally at high angles, but with local 

 western dips, apparently due to inversion. Similar rocks are, in 

 many places, exposed on the sides and the crest of the hill, present- 

 ing no appearance of decay, but hard, and often with smoothed and 

 striated surfaces. Near its western base, however, the rocks are de- 

 composed to considerable depths, as was well shown in the tunnel. 

 This, for a distance of many hundred feet, was driven in gneissic 

 strata, which, while they preserved their highly inclined attitude, 

 were so much decayed that they were excavated like earth, by means 

 of pick and spade. The brick arch, which has been constructed for 

 a distance of twenty-two hundred feet within the west end of the 

 tunnel and the stone-work of the portal, conceals, for the most part, 

 these decayed strata, but it was easy to procure specimens of them 

 just outside, where excavations were then being made in the bank, 

 exposing sections of several feet of these highly inclined beds. The 

 feldspar had been converted into an unctuous clay, which was well 

 shown in the case of coarsely granitoid layers here interstratified 

 with the more micaceous gneiss. The mica was also very much soft- 



