SECTIONAL TRANSACTIONS.—C. 398 
also in Antigua, Mexico, and other parts of the Caribbean area. The Bluff Limestone 
of Cayman Brac is therefore regarded as of Middle Oligocene age, while that of the 
_ other Cayman Islands seems, from the few identifiable fossils collected, to be of some- 
what later age, probably Miocene. Terrigenous material is absent from this lime- 
stone, and deposition in a clear sea of no great depth is indicated. 
The Zronshore Formation of consolidated coral-sand, marl and limestone 
represents the latest accretion to the area of the islands by a change in the relative 
level of sea and land. It forms a low rocky shore, known locally as ‘ iron- 
shore,’ and rises inland to a height of 12 to 15 feet above the sea, where it is 
_ generally backed by raised marine cliffs of the Bluff Limestone, which retain 
_ their verticality and freshness and show undercutting at their base by ancient 
wave-action. This coastal formation contains many well-preserved reef-building 
corals and numerous mollusea like those living round the islands at the present 
day. It is of Recent, or possibly of Pleistocene, age. A similar terrace, at 
about fhe same elevation, similarly backed by vertical cliffs of White Lime- 
_ stone, occurs on the north coast of Jamaica. 
Residual brown and red earths have been formed from the limestones of 
both ages, as in Jamaica, by the solution of their carbonate of lime. Deposits 
of phosphatised earth and limestone occur sporadically, and presumably were 
produced by the contamination of the rocks by the excreta of sea-birds. They 
are no longer worked commercially. 
The Cayman Islands may have existed as islands since Miocene times. The 
character and fauna of the Bluff Limestone show that we have in the three 
islands three fragments of the thick and widespread White Limestone forma- 
tion that extends over more than half of Jamaica on the south-east and over 
large areas in Cuba on the north, though now separated from both those 
countries by profound depressions of the sea floor. As the Bluff Limestone 
was deposited in a sea of moderate depth, and as the highest point in the 
Caymans is only 140 feet above sea-level, the elevation necessary to form the 
islands was of modest dimensions when compared with the great depressions 
north and south of the Cayman Ridge. The elevatory forces seem to have 
acted almost vertically, as no folding has been observed, but there is a slight 
axial tilt in Cayman Brac. Aerial denudation has had time since elevation 
took place to convert the surface of this upper platform into a matured ‘ karst’ 
country, now thickly forested. The latest important change of level was that 
which added the coastal platform to the islands in Recent or Pleistocene times. 
As similar low terraces are found in Jamaica, Cuba, and other parts of the 
Caribbean region, the change of level seems to have been widespread, involving 
more than one tectonic unit, and may therefore have been due more to a fall 
in sea-level than to earth movement. Around the shores a shallow submarine 
platform extends about as far as the 10-fathom contour, beyond which the 
depth increases rapidly, and on this platform the modern coral reefs grow. 
The author has received palxontological assistance from Dr. T. W. Vaughan. 
5. Prof. O. T. Jones.—The Ordovician-Silurian Boundary in Britain 
and North America. 
Owing to various causes American geologists find difficulty in correlating the 
rocks near the Ordovician-Silurian boundary with the divisions established in 
Britain. Some of these difficulties are due to early stratigraphical errors, 
others may be ascribed to imperfect descriptions of British Paleozoic faunas, 
and others are due to differences between the facies of the American and British 
‘rocks. The author classifies the Upper Ordovician and Silurian rocks into 
three facies—pelitic, psammitic, and calcitic—according to their situation rela- 
tive to the shore lines of the periods, and on the basis of the distrbution of 
brachiopod and trilobite genera attempts a correlation of North American 
_ divisions with British rocks. 
_ As regards Anticosti, the author is in agreement with Twenhofel in corre- 
lating the English Head, Charleton and Ellis Bay formations with the upper 
part of the Bala and the Becsie River, Gun River, Jupiter River, and Chicotte 
formations with the Silurian. These are mainly equivalent to the Upper 
Llandovery, but the Lower Llandovery appears to be poorly represented by the 
Becsie River formation. 
