ESSEX COUNTY. 



239 



the most remarkable of these phenomena. JIany facts of an interesting nature, and having 

 an important bearing on tiie origin of the magnetic oxide, will, williout doubt, be brought to 

 hght in the farther exploration of several of these veins ; and I am particular in pointing to 

 this locality, as one which will be peculiarly interesting to American geologists. As yet the 

 phenomena connected with veins of iron have not received that attention which they deserve 

 in a geological point of view. They have been visited more for mineralogical purposes, than 

 for the study and observation of the peculiar plienomena which attend liiem. 



The Hall vein is the first which I shall notice ; it is five feet wide, and dips west at an 

 angle varying from fifteen to twenty degrees. The vein does not correspond in dip to that of 

 the strata ; it ranges, however, about east-northeast. The ore is black, but not bright ; it is 

 mixed with grey quartz, and a large proportion requires separating, or washing. The walls, 

 or rather the roof, is smoothed and striated by pressure and friction, or it exhibits that appear- 

 ance termed slickensides — an appearance produced by the forcible propulsion of a very heavy 

 body across a moderately even surface. The wall which is most exposed projects in upon 

 the vein, exhibiting an imperfect serrated appearance, as in the following cut : 



62. 



a, vein of ore ; 6. rock projecting in upon the vein. 



Those masses of rock which thus project in upon the vein, are frequently quite large ; how- 

 ever, they disappear in the progress of the work. 



The roof or superior wall in the Hall vein is quite irregular, and it is possible that it is not 

 the true wall. If this conjecture is true, the rock will finally disappear, and another mass of 

 ore will be found on the west side. In fact, at some of the openings along the vein, such 

 a result appears very likely to follow ; as on exploration a few feet west, ore is found which 

 appears to be separated from what seems to be the main vein, by a layer of rock. 



It is sometimes found that layers of rock project nearly across the vein, in a manner repre- 

 sented in fig. 62. 



This section of the vein is exposed at the most southern point where it was opened. I was 

 told that at first those masses of rock projected entirely across the vein ; and that at the depth 

 of five feet, they had diminished in length nearly one-half; so that there is very little doubt 



