170 



THE GEOLOGIST. 



It is a conspicuous object for a great distancGj and resembles a tower 

 in ruins.* 



14. Thunder Mountain and Pate Island Pilasters, Lake 



Superior. 



Thunder Mountain, several miles long, rises from the eastern 

 angle of Thunder Bay, and is fourteen hundred feet high as measured 

 by Count Adriani. The west half of its summit is almost tabular ; 

 whilst the eastern half is irregular and hummocky, dipping suddenly 

 in round masses, into a lower but still elevated country. About the 

 middle of its south side, where the height is greatest, an immense 

 cavity, with steep woody acclivities, is scooped out of the body of the 

 mountain. The upper third of the elevation in the south-west is 

 occupied by precipices, fissured into vertical pilasters, weathering 

 orange red, and occasionally advancing in the form of large but- 

 tresses. These precipices are very extensive. The pilasters are 

 smooth prolonged perpendicular slabs, formed by the disappearance 

 of vertical slips of rock, at certain intervals. t 



The tower-like eminence, fourteen hundred feet in perpendicular 

 height at the west end of Pate Island, some miles distant, is flat- 

 topped, and its sides are faced with vertical pilasters resting on a 

 talus, like those of the Thunder Mountain. These pilasters have 

 been compared to basaltic columns in the distance, with an apparent 

 but not real horizontal stratification. " In some places they have 

 fallen out, leaving hollows like flues in the side of the cliff. In other 

 places single columns stand out alone, like chimneys ; in others 

 again, huge flat tables of rock have scaled off from the face of the 

 wall. "J Trappose greenstone is the prevailing rock from Thunder 

 Mountain westward, and gives rise to the pilastered precipices of 

 Fort William. 



All the foregoing (ISTos. 12, 13, and 14) are formed by the rocks 

 bclongiug to the Huronian system of Sir William Logan, Avhich 

 consist of slates, sandstones, limestones, and conglomerates, with 

 immense masses of greenstone interstratified. These repose uncon- 

 formably upon the Laurentian rocks. The Grange is composed of 

 greenstone, as well as many of the low islands of the Mammelles and 

 others, which have become hallowed by the waves into boAvls, caves, 

 and small arches. Many of the rude colonnades are formed of por- 

 phyry, which plunge into the lake, or crown the highest summits, 

 and occasiounllv they are fissui^ed. The pallisades of Thunder 

 Mountain arc> a s;reenstone trap. 



In describing the geological structure of Maimanse, the most 

 eastern promontory on the shores of Lake Superior, Dr. Dawson of 



* Cooi:;ra])hy ai\d Geology of Lake Superior, by Dr. Big-sby, Tran. Geol. Soc, 

 per. 2, vol. i. 

 t Idem. 



X Ag-nssi/, T-nk(^ Suporior, p. 93. 



