'200 Dr. BiGSBY on the Geography and Geology of Lake Huron. 



lar stratum occurs, at the level of the lake also, at the north-west end of the 

 Little Manitou, and at the south-west angle of the Grand Manitou. In the 

 Little Manitou it contains anomi£B, which abound at the south-east angle of 

 the island. 



On proceeding- inland, at CoUier's Harbour, at an elevation of 100 feet, we 

 find ledges of very hard quartzose white or light brown limestone, piercing 

 the sides of the slope, and finally, in somewhat slaty and squared blocks, con- 

 tributing to form Blockhouse Hill. The fractured surface of this rock sparkles 

 with what the frequent crystallizations in cavities and fissures show to be quartz. 

 It is in this neighbourhood that the nondescript madrepore hereafter to be 

 described is principally found. The angle of Drummond, on the north of the 

 False Detour, and part of the western end of the Grand Manitou, rise in cliffs 

 250 feet high. The rock, in both cases, resembles the quartzose limestone of 

 Collier's Harbour. Major Delafield found the summit of the former to be a 

 platform, crowded with ammonites and anomiae. The latter, to judge from 

 its debris, contains the same organic remains as Blockhouse Hill. It abounds 

 with astreae, and chain-coral, 2 or 3 yards in diameter. Its fissures are fre- 

 quently lined with honey-yellow talc-spar. To the south of Drummond is an 

 isle, almost wholly composed of iron pyrites, mixed with some brown calcare- 

 ous matter and quartz nodules. 



In parts of the Little and of the Grand Manitou, the limestone on the shore 

 is of a blueish gray or light brown colour, very finely granular, and extremely 

 slaty. It divides into small fragments no thicker than paper, which are ar- 

 ranged by the wave, edgeways, in undulating lines round the boulders on the 

 beach. I could not find in it any organic remains; but trilobites have been 

 met Avith in a rock so similar, found in loose fragments on the shores of Drum- 

 mond, that I believe them to occur imbedded in this limestone-slate. 



Immediately above this slaty bed, or alternating with it, in both Manitous, 

 is a stratum, which occurs in thin broken patches, and is brown, and almost 

 black in parts. It is very soft, of coarse texture, and full of knots of all sizes. 

 These knots in the Little Manitou are very fetid, and often send off several 

 short round branches. They often bear strong marks of cellular madrepore. 

 This is by no means so slaty as the contiguous bed. It abounds in organic 

 remains, some of which are peculiar to it : others are common to the Mani- 

 toulines. 



In the middle of the north side of the Little Manitou, 30 miles east of Col- 

 lier's Harbour, and 100 feet above the level of the water, in low interrupted 

 cliffs, we find a dull-brown granular hmestone, rather hard and slaty, fetid, 

 and free from shells. We have the same rock at the same level in the Grand 



