Islands of Group III.
The large islands of this group are mainly built of Silurian limestones. North Somerset was the only island of this group visited in the Neptune, and all information concerning the others is derived from the observations made on earlier expeditions, and contained in Dawson’s summary of the northern geology.
Silurian limestones form the southern third of Banks island, being overlaid in the northern part by beds of Devonian and lower Carboniferous age. Dr. Rae reports the entire southern coast of Victoria island as being composed entirely of Silurian limestone.
‘The northern part of King William land, with Matty island to the east of it, are described by Sir John Ross as of limestone. Simpson states the eastern part of the south shore to be also of limestone, and Haughton dealing principally with the results of M’Clintock’s voyage writes as follows: “The east side of King William island, though composed of Silurian limestone like the rest of the island, is strewed with blocks of black and red micaceous gneiss, like that of Montreal island, and black metamorphic clay-slate, in which the crystals of mica are just commencing to be developed. It is probable that the granitoid rocks appear at the surface somewhere to the eastward of this locality.”
‘Numerous excellent though brief notes on the geology of the eastern and southwestern coasts of the Boothian peninsula occur in Sir John Ross’ remarkable narrative referred to. From these we learn that the eastern shore is composed of limestone to Port Logan (latitude 71° 21´), where a high range of hills—which is seen at a distance estimated at thirty miles inland at Creswell bay (further north) and runs north-and-south—impinged on the shore, and was found to consist of granitoid and gneissic rocks. Thence southward, from notes given in the body of the narrative, a narrow border of limestone may skirt the shore to about latitude 70° 35´, though the geological appendix does not make any mention of this.’
‘The narrow neck of the Boothian peninsula, which was crossed by Ross on several lines, is, from his description, composed of granitic rocks, with some outliers of limestone. One of these, definitely mentioned in the narrative but not in the geological appendix, is shown on the present map. On the coast of the mainland, west of the isthmus, the limestone formation is found resting on the granites of Lake Wittersted. Northwest from the isthmus the southwest coast of Boothia presents a range of granitoid hills, running northward, but becomes fringed by a low border of limestone near Cape Isabella, and this increases in width to the north, till an extensive flat limestone region is found in the vicinity of the magnetic pole.’
At Bellot strait a junction occurs between the granite and horizontal beds of Silurian limestone.
‘The entire western portion of Prince of Wales island is composed of Silurian limestone, which in the extreme west, at Cape Acworth, becomes chalky in character and non-fossiliferous, resembling the peculiar Silurian limestone found on the west side of Boothia Felix.’
The northern and eastern shores of North Somerset are of limestone, usually rising from the water in precipitous cliffs. These were examined at Port Leopold, where the cliffs rise sheer 1,000 feet from the sea. The bedding of the limestone is very distinct, and the face of the cliff has been sculptured by every runlet, so that between the horizontal and vertical markings the cliffs resemble on a gigantic scale the fluted walls of a castle. Fossils were collected from the lower beds of the cliff by M’Clintock, but none were collected on the present voyage.