The boulders of ore as mined vary from sizes as small as peas up to masses weighing 100 pounds and more; all of these are well rounded.

It is hardly necessary to state that there are no deposits which rank higher than this of Iron mountain. If accessibility and ease of mining be taken into account, its competitors must still more be reduced. The early development was done when iron was not in great demand. Thus a large part of the deposit was mined in time to take advantage of the brisk markets for ore between 1870 to 1880. In this decade many thousands of tons were sold at $10 per ton and orders were filled at $15. Fully 66% of the ore at Iron mountain was mined between 1870 and 1890, and the greater part of this from 1870 to 1886. The composition of the ores of Iron mountain presents rather a wide range as to the amounts of phosphorus, as the following analyses show:

Metallic iron.....

Phosphorus

.....

1 2 3 4 5 66.93 59.06 65.57 64.67 62.84 0.071 0.398 0.032 0.019 0.005 0.106 0.674 0.049 0.029 0.008

Phosphorus ratio.....

No. 1 is from the main vein; No. 2, from same vein nearer the porphyry, and contained visible crystals of apatite; No. 3 is a sample of surface ore; No. 4 is from the surface near the main vein on Big mountain; No. 5, from the surface of Little mountain.

The silica is rarely high; the highest given is 11.68%, the lowest 2.51%. The above partial analyses are fairly representative.

It is a little remarkable that the smaller veins and the part of the large veins near the contact with the porphyry walls contain the highest percents of phosphorus. Why the solid veins should contain the highest phosphorus is more easily understood. Many places in the more exposed parts of the veins are honey-combed with cavities from which crystals of apatite have been removed by weathering, and, on following such a surface downward to a point remote from weathering influences, quite fresh crystals are found. From the boulder ore, both of the surface and the lower Paleozoic, almost the last vestige of phos phorus has been removed, while the silica is remarkably low. At the point where the veins begin to give out, the ore grows lean and sili cious through the admixture of porphyry and of quartz crystals and grains which are included in the vein matter.

As the map shows, numerous bore-holes have been put down, which reached depths of from one to three hundred feet. These usually start in a net-work of small veins of ore. As depth is gained the

veins grow fewer as the fresher porphyry is reached, and, though they have not wholly disappeared in every case at the bottom of the holes, they seem to prove conclusively that no rich veins lie below. These holes have been put down so systematically and carefully that it is hardly possible that a rich and extensive deposit, either vein or detrital, could have escaped detection. It seems, therefore, a fact, though much to be deplored, that these test-holes point to the conclusion that the glory of Iron mountain is of the past.

BUFORD MOUNTAIN.

About one mile west of Iron mountain is a large porphyry hill known as Buford mountain (shown on the map). Considerable "float" ore has led to the discovery of small and unimportant veins of iron in the porphyry. No work of development has been done, as the place does not seem to be of sufficient promise. It is expected, however, that a series of drill-holes will be extended across from Iron mountain to Buford. These prospect holes will reach into the porphyry through the overlying Paleozoic rocks of the intermediate valley. If the veins which show up in Buford mountain enlarge in the valley it is hoped thus to strike them. On the other hand, if the known small veins are but the remains of once greater veins, it is thought that conglomerate deposits will be found similar to those at Iron mountain.

PILOT KNOB.

This famous deposit is situated six miles south of Iron mountain. As is shown in the illustration on the next page, it is in a nearly conical hill rising 1521 above tide and 662 above the surrounding valley. The knob stands almost completely isolated from the surrounding porphyry hills, being connected on its eastern side by a neck of porphyry only 200 feet above the surrounding valley. Unlike the porphyry hills in general, the rocks of this hill are distinctly stratified near the summit. The beds have a dip of about 13° to the southwest. It is in this distinctly stratified part of the knob that the deposit of specular ore is found. The cross sections of the section sheet accompanying this report show very well the relations of the ore body to its enclosing rock, while the topographic features of the knob are shown in the cut of the opposite page constructed from maps prepared by Prof. Potter. There are comparatively few exposures of the porphyry below the outcrop of the orebed. The surface of the hill is covered by a heavy mantle of the

Fig. 14. A contoured map of Pilot Knob and vicinity.

The cross-lined area shows the extent of mining on Pilot knob. Small circles with black centers are drill-holes; the accompanying numbers are those of the Pilot Knob company.

Datum of contours is sea level.

Scale is 2500 ft. 1 in.

residuary clay. This clay is of two kinds: first, the clay which is the residuum of the decomposing porphyry, and second, the residuary clays of the Paleozoic limestones which once reached high up the sides of the knob.

Scattered over the surface, so thickly as at times to wholly obscure the soil, are fragments of porphyry of various sizes. These porphyry fragments are, in places, thickly sprinkled with boulders and fragments. of specular ore which have become detached from the parent lode, and have rolled down the steep sides of the hill. If a pit be dug in these surface materials to the solid rock, it will be found that the rock and ore fragments are imbedded in the clay as well as scattered about on the surface of the hill. On the northwestern slope of the hill the ore fragments constitute such a large per cent of the entire mass that it has been found profitable to separate the ore. This fragmental ore is traced more or less abundantly down the slope to near the limestones, and bore holes through the limestones in the narrow valley between Pilot knob and Buzzard mountain seem to show that this detrital deposit extends under the limestones as do the detrital deposits at Iron Mountain.

Ascending the uniformly steep grade of the hill to about 150 feet of the summit, we come to the deposits which have given the mountain its reputation. At this point the clays and detrital covering cease and we are led by a series of precipitous rocky bluffs to the summit of the knob.

The first rocks which crop out below the ore-body are unmistakably of a porphyritic nature. At first sight they seem to differ in no material respect from the other porphyries in this region; but closer inspection, however, shows them not to be perfectly homogeneous, but to be made up of fragments of various sizes and shapes and to be characterized by bedding planes. As we approach the ore-body these bedding planes become more and more distinct till the porphyry characteristics wholly fade and are succeeded by alternating beds of siliceous and ferruginous slate, the silica being in the form of more or less rounded grains of sand, and these with thin bands of good ore. The foot-wall proper of the ore-body is marked with rugose bands having the appearance of ripple-marks, which they undoubtedly are.

Next in order of upward ascension is the ore-body proper. This, as mining operations have shown, is from 19 to 29 feet thick. It is remarkably homogeneous, having only occasional slaty siliceous seams

or thin beds, and has in a slight degree the variations in thickness which are characteristic of any bedded deposit. The ore body itself is decidedly slaty in structure, with numerous jointing planes. Lying immediately over the ore body, into which it occasionally passes by insensible gradations, is a bed of ferruginous slate. In places this slate is only a few feet thick, but toward the eastern side of the ore body it thickens at the expense of the ore, which thins out to a small bed. The opposite cut gives an idea of the appearance of the slate. Still ascending, we note that the slates gradually give way to a more thickly bedded deposit, which, instead of the fine grain of the slate, partakes more of the nature of a conglomerate. The conglomerate characteristics increase till, within a few feet above the slate, the beds consist wholly of a coarse breccia, which holds its own to the summit of the knob. There is much iron present in the matrix of this conglomerate; in fact a good part of it could be used as a low grade of ore, were it not for its association with coarser material.

The general appearance of the knob being thus described, we can now take up the more detailed description of the iron ore body. The ore of Pilot knob differs very essentially in its physical aspects from the ore of Iron mountain. It will be remembered that the ore of Iron mountain is very coarsely crystalline, though compact and dense. The Pilot knob ore, on the contrary, is fine grained, hardly ever showing a trace of the coarse crystal faces of the other, and is just as compact and dense as the Iron mountain ore. Another material point of difference is in its structure. The Iron mountain ore is massive—that is, it breaks like granite-while the Pilot knob ore has more of a slaty structure, which causes it to split quite readily. This peculiarity is more plainly noticeable in the detrital ores which are found in the talus on the northwestern slope of the knob. This ore, on weathering, breaks very readily along planes which are scarcely discernible in the ore frechly mined from the bed. Another peculiarity is in the silica contents which are very evenly disseminated throughout its mass. Phosphorus, a very undesirable element, is also present in remarkably small quantities, and is uniformly distributed through the entire ore body. The following partial analyses of the Pilot knob ores show this even distribution of phosphorus. No. 1 is of the highest grade, and No. 2 of the lower grade of ore as shipped: