TACONIC QUESTION IN GEOLOGY, 261 
veries of Dale, Dwight and others, that still newer fossiliferous strata, of Ordovician age, are 
also included in this part of the Hudson valley ; and that we have, in fact, in this region the 
three groups of rocks long since pointed out by Emmons. (§108) The testimony of Logan 
is valuable as confirming that of Emmons, and of Hall, as to the existance of portions of 
the Second Graywacke series resting, not npon the Trenton limestone, but upon the older 
schistose rocks of the region; and moreover, as showing the super-position of the First 
Graywacke to the Taconian. 
§117. The apparent absence of the characteristic limestone of the Trenton from the base 
of the Second Graywacke in this region may be due to a stratigraphical break and erosion 
at the close of the Trenton period, as we have seen in the Ottawa basin. Two other 
explanations are suggested by the thinning-out of the limestone-mass to the south-east, as 
already noticed (§111). One, that the region was beyond the Trenton sea, and the other 
that its sediments, over the area in question, were argillaceous beds, resembling rather the 
succeeding shales than the limestone deposited elsewhere. That this latter was the case in 
parts of the eastern region, will be shewn in the sequel, but we shall there also find many 
evidences of movements in paleozoic times subsequent to the deposition of the Trenton. 
I have elsewhers pointed out, (Azote Rocks, page 123), besides the post-Trenton break in 
the Ottawa basin, the evidence in eastern North America of not less than five epochs, 
marked by movements of the strata, and by unconformities, subsequent to the deposition 
of the Trenton and Utica divisions. Ofthese the earliest, and the only one which now con- 
cerns us, is that of which we see evidences in the unconformable superposition of Silurian 
beds over older strata to the north and east of the Hudson valley. On St. Helen’s Island, 
near Montreal, we find reposing on the eroded surface of the slightly inclined Utica 
slates, a portion of fossiliferous limestone, associated with a dolomitic conglomerate. The 
faunain the limestone is referred to the age of the Lower Helderberg; while the accompanying 
conglomerate contains forms which belong rather to the Clinton and Niagara divisions of 
the Silurian, and holds at the same time pebbles of fossiliferous Trenton limestone, with 
others apparently of Potsdam sandstone, Utica slate, and red sandstone and shale, resembling 
those of the Medina ; the whole mingled with pebbles of Laurentian gneiss, and of igneous 
rocks, giving evidence of a period of disturbance, and considerable erosion of the 
older rocks. Other masses of similar conglomerate are found elsewhere in the vicinity, in 
one case holding Silurian fossils, resting on various members of the Champlain division, and 
on the Laurentian gneiss. 
Another mass of Lower Helderberg limestone is met with on the flanks of Belceil Moun- 
tain, an eruptive mass which breaks through the Loraine shales in the Richelieu valley. 
In the distribution of these, and of similar areas of fossiliferous limestones, we have the 
evidences of a Silurian sea, which extended from the Helderberg region in New York, not 
only through the valleys of the Hudson and Lake Champlain to that of the St. Lawrence, 
but also through those of the Connecticut, the St. Francis and the Chaudière, and thence 
to Gaspé ; depositing its sediments, with their characteristic fauna, unconformably over rocks 
of very different ages. We have similar evidence that the Trenton-Loraine, or Ordovician 
sea had already, in like manner, extended over parts of this region, leaving its fossiliferous 
sediments spread unconformably over Cambrian and pre-Cambrian strata. 
§ 118 A section from Crown Point, New York, across the southern part of Lake Cham- 
_ plain eastward to Bridport, Vermont, which was studied in detail by Wing, and by 
