586 [April 2. 



almost always containing hornblende, either as a fourth constituent 

 mineral, or as replacing the mica. Frequently the mica and quartz 

 are both absent, and the rock becomes a syenite, which, by a gradual 

 diminution of its felspar, passes into crystalline greenstone, and 

 whilst in some parts there is thus a complete passage from granite 

 to greenstone, in others the different varieties alternate in immense 

 beds, whose general direction is to the N.E. Basaltic dykes, often 

 of enormous size, traverse these beds, sometimes passing from one 

 to another unaltered, and ranging up the hills for miles. 



Associated with these rocks, penetrating them in veins and 

 dykes, or reposing on them, are various trap-rocks, occurring on 

 both sides of the lake and on its islands, but most extensively 

 developed on the northern coast. They consist of various por- 

 phyries and amygdaloids, syenites, greenstones, and basalts, the 

 last being occasionally prismatic, with pitchstone in thin layers 

 between the interstices of the columns. Many of these rocks pass 

 into each other, forming endless varieties. 



In the primitive rocks, besides the usual constituent minerals, 

 including hornblende, we find schorl, garnet, amethyst and rock 

 crystal ; epidote and purple fluor spar associated in veins of quartz 

 in granite ; chlorite and green earth, with calc-spar in veins, 

 in granitic and greenstone rocks ; sulphate of barytes, with fluor 

 and calcareous spars in a vein of greenstone. The magnetic black 

 and brown oxides of iron, specular and micaceous iron, and iron 

 pyrites, occur abundantly ; copper pyrites rarely, in veins in 

 greenstone. 



The trap-rocks are rich in minerals. I noticed calcedony, 

 carnelian, jaspers, various and beautiful agates, zeolites (mesotype 

 and stilbite), epidote, augite, olivine, green- earth, fibrous prehnite, 

 fluor-spar, satin-spar, calc-spar, amethystine quartz, and felspar 

 crystals ; graphite, and the oxides of iron before mentioned ; also 

 copper pyrites, malachite, and native copper. The last occurs, 

 with prehnite, quartz, and calc-spar, in veins in a dark brown por- 

 phyritic trap ; also in amygdaloid in veins, nests, and wires, which 

 sometimes penetrate fortification agates. 



Reposing on the granitic rocks which have been mentioned, there 

 occurs a horizontal sandstone, forming nearly the entire southern 

 shore of the lake, which may be traced in detached portions from 

 one end to the other of its northern shore, and also on most of its. 

 islands ; the almost unknown Caribou Island, which is so far from 

 the shore that it can only be seen from the mountains in a clear 

 day, being also formed of it. It may, therefore, be considered as 

 a general formation over the basin of Lake Superior. It rises 

 to about the height of 400 feet above the lake, and rests on 

 granite on either shore, excepting where amygdaloids or other 

 traps may have been intruded between. It has been only occa- 

 sionally upheaved by the granite ; but it is often much shattered 

 by the trap-rocks, nodules of which enter largely into the compo- 

 sition of its conglomerates. It is often extensively overlaid, 

 especially in Mepigbn Bay, by immense beds of greenstone several 



