No. 3.] ROBE — GEOLOGY OF ST. IGNACE ISLAND. 175 



The amygdaloid trap, although uot of seJimeutary origin, in 

 the ordimry acceptation of the term, is regularly bedded, hav- 

 ing a very distinct dip to the south, at an angle of ab »ut 13" ; 

 and is overlaid, at numerous points, by a compact, very hard and 

 heavy, finuly-erystalline trap or greenstone, or it may be diabase 

 or basalt. Which of these is the more correct term, I confess I 

 am not sufficient of a mineralogist to determine, nor could it be 

 accurately determined without an ;in ;lysis. It is probablv com- 

 posed of the same mineral ingredients as the body of the amyg- 

 daloid trap, but with a very difierent texture and appearance; 

 and is entirely devoid of the characteristic amygdules or small 

 rounded masses of foreign minerals which occur so copiously iu 

 tbe other rock. This overlying rock is, I believe, tliat which, 

 by tiie South Shore miners, is always designated as Greenstone, 

 and it is there well understood that it is generally at or near its 

 junction with the underlying amygdaloid trap that the produc- 

 tive metalliferous veins or deposits are to be found. In the 

 present case it overlies the amygdaloid in numerous isolated 

 knobs, patches and ribs, distributed along the eastern shore of the 

 location, and some of the small outlying islands, standing out in 

 bold precipitous bluffs and precipices, sometimes about 100 feet 

 high, plunging into the deep waters of the lake; the intervening 

 spaces being excavated by the waves into deep bays terminated 

 by gravel beaches. 



In some instances the masses of crystalline trap or greenstone 

 are abruptly termin;.ted downwards, at or very near the level of 

 .the lake; and their planes of junction with the amygdaloid are 

 distinctly visible at that point, maintaining the regular dip of 

 the amygdaloid beds, although the greenstone itself shows no 

 tendency 10 a bedded structure. In other instances there appears 

 to be a sort of passage between the two — I mean only in so far 

 as their distinctive mineral characteis are coiicerned. In two 

 places the greenstone assumes a b.isaltic columnar structure ijoth 

 vertical and horizontal; and the surfaces of the rock^ are there 

 sometimes found to be coated with pitchstone, or p'^rhaps the 

 rare mineral tachylyte, as suggested to me by Dr. Harrington. 



By reason of the hardness and extremely refractory nature of 

 the crystalline trap, the masses exposed on the shores of St. 

 Ignace Bay stand up conspicuously above the general level of 

 the amygdaloid, with steep mural faces to the north; forming 

 ridges running inland in a due. westerly direction, conformably 



