6o4 THE POPULAR SCIENCE MONTHLY, 



about five inches in diameter, though some are eight and even ten inches, 

 and of a rounded oval outline. They were certainly gathered as peb- 

 bles along the shore of the lake, north from the island, where there are 

 still others of the same shapes and sizes, and of the same varieties of 



Fig. 4. Raw-hide String and Knot or the Isle Rotale Ancient Miners. 



rock, formed on the beach by the action of the waves. The great 

 profusion in which they are scattered among the debris of the pits 

 would itself indicate the ease with which they were obtained. They 

 are not grooved for the reception of a withe, like those found on the 

 south shore, near Ontonagon, but they were apparently used for the 

 most part by simply swinging them in the hand, or probably in both 

 hands clasped, thus by repeated blows breaking away the surrounding 

 rock or hammering the desired metal into such shapes as to facilitate 

 its separation in smaller pieces. The rock of which they are composed 

 does not occur as pebbles on Isle Roy ale, and indeed it is doubtful if it 

 exists at all on the island. It forms the coast of the mainland for sev- 

 eral miles opposite the island. It is an igneous rock, visually a dia- 

 base, as shown in thin sections under the microscope, consisting essen- 

 tially of a triclinic feldspar and augite, with magnetite. Sometimes 

 the grains are coarser, and the rock would more properly be styled a 

 dolerite or a gabbro. They belong to the formation designated by Sir 

 William Logan Tlie Lower 'Volcanic Group, but since styled Animihie 

 Group, by Professor T. S. Hunt.* Occasionally, however, the work- 

 men seem to have gathered rounded stones of other varieties of rock, 

 though nothing equaling the firmness of the above, and so fit for the 

 purpose of a rude hammer in simple mining, can be selected among all 

 the rocks of the region. One or two, of a granite containing red or- 

 thoclase, were seen at the mine, and a few of other granites are report- 

 ed to have been found. These other varieties are also seen mingled 

 sparsely with the diabase stones along the Canadian shore, and are 

 referable to the drift forces which transported them from farther 

 north and east in Canadian territory. 



Although these hammers, as a rule, are not withed, it is still true 

 that occasionally one is found that is withed i. e., grooved for the re- 

 ception of a withe handle. One seen at the time of this visit was owned 

 by Dr. Gailey, and was not well wrought. The groove was evidently 



* Vide " Trap Dikes and Azoic Rocks of Southeastern Pennsjivania," " Second Geo- 

 logical Survey of Pennsylvania," pp. 68, 240. 



