22 Captain Bayfield on the 
on this lake. There is however a large tabular mass, or per- 
haps a bed of crystalline limestone, (discovered by us,) super- 
imposed upon, or imbedded in the trap greenstone of the slate 
islands. ‘The redrock above mentioned is also abundant in 
the western parts of Neepigon bay. It has been supposed that 
it is stratified, but this we think is amistake arising from the 
stripes and veins which we have mentioned. Doctor Bigsby 
has ventured an opinion, founded principally upon the above 
supposilion, that this is porphyry divested of its crystals, and 
also that it passes into sandstone.* 
The first part of this supposition we think extremely pro- 
bable ; but the last, we think, would require strong and un. 
answerable reasons to support it, ere it could be received as 
an undoubted fact, contrary asit is to all former observations 
on the nature and connexion of those rocks. We have men- 
tioned the colour of this rock. It is of different degrees of hard- 
ness in different places ; sometimes as hard, or nearly so, 
as hornstone,when it has a flinty fracture passing into conchoi- 
dal. At others it is slightly granular. It also occurs of earthy 
fracture, and sosoft that it can be cut by a knife, in which 
state it is used by the Indians to make their pipes. It then 
very much resembles potstone. Upon the whole we are of 
opinion that the base of this rock is cempact felspar, and that 
it is nearly allied to the porphyries, 
Having now noticed the whole of the rock formations of 
Lake Superior, we shall give a brief description of the large 
islands, which have, we believe, never been visited with this 
object, by any excepting ourselves. 
iS. Le Royale is about 45 miles long, by 7 or 8 broad, ly- 
ing ina north east and south west direction, Its centre is 
composed of ridges of trap sienite parallel to its length, and of 
greenstone, which last abounds principally on its south side. 
*Sce Doctor Bigsby on the geology and geography of Lake Su: 
perior in the periodical work mentioned in a former note. 
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