tense pressure, as evidenced by the extensive cataclastic structure 

 which has been developed in both series of rocks. Frequently the 

 rocks show a pyroclastic origin, and volcanic tuffs and breccias are very 

 commonly met with. The relations of the diabase or basic irruptive 

 rocks with the surrounding sedimentary strata was closely examined in 

 a large number of instances, and revealed the fact that the diabase is 

 apparently of later age, as it breaks through and alters the bedded 

 Huronian. The occurrence of these masses of diabase with a sur- 

 rounding breccia or agglomerate in many cases would seem to point to 

 the fact that they are the bases of Huronian volcanoes, which continued 

 in action after the latest sediments had been deposited. Some of these 

 diabasic masses send out dykes which ramify through and alter the sur- 

 rounding strata, these dykes frequently containing fragments of highlV 

 metamorphosed Huronian quartzite. These irruptive masses are 

 usually lenticular, although occasionally rudely circular or oval in out- 

 line, and their longer axes correspond in general with the strike of the 

 enclosing rock. They vary in breadth from a few chains to half a mile, 

 or even more, and frequently extend for miles in length. The origin ot 

 the nickel and copper is closely connected with this diabase or gabbro, 

 and the formation of the fissures containmg these ores was no doubt 

 due to the disruptive forces of the intrusion, and the contraction caused 

 by the subsequent cooling of the igneous rock matter. These fissures 

 were necessarily most frequently formed along the line of contact with 

 the cooler sedimentary strata although in certain cases they were formed 

 in the midst of the igneous mass itself. In nearly every case, therefore, 

 the deposits of nickel and copper occur close to the contact of the 

 diabase with the stratified rocks, although in a few cases they are found 

 in the diabase near its junction with granite or micropegmatite. An- 

 other proof of the common genesis of these ores and the enclosing 

 diabase is that the diabase itself commonly contains these sulphides 

 dissemmated through its mass, these impregnations occasionally form- 

 ing such considerable and rich deposits as to be workable. 



All geologists who have examined these deposits agree that they 

 are not true fissure veins, and although at times a certain sloping sur- 

 face is obtained which seems to have a uniform inclination, yet it 

 seems certain that there are no regular walls in the miner's sense of the 



