170 
THE GEOLOGIST. 
It is a conspicuous object for a great distance, and resembles a tower 
in ruins.* 
14. — Thunder Mountain and Pate Island Pilasters, Lake 
Superior. 
Thiander 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 iri'egular 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 accli%aties, is scooped out of the body of the 
mountain. The upper third of the elevation in the south-west is 
occupied by pi'ecipices, fissured into vertical pilasters, weathering 
orange red, and occasionally advancing in the form of large but- 
b-esses. These precipices are very extensive. The pilasters are 
smooth prolonged perpendicular slabs, formed by the disappearance 
of vei"tical slips of rock, at certain intervals. t 
The tower-like eminence, fourteen hundi'ed 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 ]\Iountain. These pilaslers have 
been compared to basaltic columns in the distance, mth an apparent 
but not real horizontal stratification. " In some places they have 
f;\llen out, lea%-ing hollows like flues in the side of the cliff. In other 
places single columns stand out alone, like chimneys ; in others 
ag'ain, hug-e 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 (Nos. 12, 13, and 14) are formed by the rocks 
belonging to the Huvonian system of Sir William Logan, which 
consist of slates, sandstones, limestones, and conglomerates, with 
immense masses of gi'eenstone interstratified. These repose uncon- 
formably upon the Laurcntian rocks. The Grange is composed of 
gi'eenstone, as well as many of the low islands of the Mammelles and 
others, which have become hallowed by the waves into bowls, 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 occasionally they are fissui-ed. The pallisades of Thunder 
Motmtain are a greenstone ti'ap. 
In describing the geological structure of Maimanse, the most 
eastern pi'omontory on the shores of Lake Superior, Dr. Dawson of 
* Geography and Geology of Lake Superior, by Dr. Bigshy, Tran. Geol. Soc, 
ser. 2, vol. i. 
t Idem. 
J Agassiz, Li)kc Supoi ior, p. 9.3. 
