464 
view of determining the amount of dislocation which the enclosing 
beds may have undergone, and of deciding in some cases whether the 
strata have been completely inverted; but considerable caution, the 
author says, is necessary in the application of this test, and that the 
inference must be drawn, not from a single specimen or a few corals 
being reversed or inclined, but from the prevailing disposition of the 
great masses. At Gleedon Hill and Bradley, near Wenlock, he no- 
ticed that some of the Polyparia, particularly beds of Catenipora, main- 
tained their original vertical direction, while others were inclined or 
reversed and mingled with broken stems of Crinoidea, leaving no 
doubt upon his mind that the dislocated specimens were fragments 
which had been broken off by the action of the waves, and thrown 
down upon the reef. 
From the inquiries of Mr. Darwin and other naturalists, it appears 
that stone-corals do not flourish at a depth exceeding 120 feet. 
Without assuming that the habits of extinct species were precisely 
similar to those now living, Mr. Lyell says, it may nevertheless be 
inferred from analogy, that the stone-corals of the Silurian period 
did not live at a depth of many hundred feet; and, consequently, 
that those parts of the Wenlock limestone in which the corals pre- 
serve their natural position, were produced at a moderate depth 
from the surface. This conclusion, he shows, is also supported by the 
occurrence of the inverted and broken corals noticed above, and as- 
sociated with others in the position in which they grew. 
A further inference drawn by Mr. lyell from the limited depths 
at which corals grow beneath the surface of the ocean, is the sub- 
sidences which must have consequently taken place during the accu- 
mulation of the upper Silurian strata. Thus in the Gatley escarp- 
ment near Aymestry, he shows, that the lower or Wenlock coralline 
limestone is separated from the upper or Aymestry limestone by 
more than 400 feet of mudstone or lower Ludlow strata, and that 
in the same neighbourhood a great thickness of mudstone, amounting 
at the New Bridge, Ludlow, to 700 feet, is superimposed on the Ay- 
mestry limestone. It is, therefore, evident, he says, that at least 
two great subsidences took place during the accumulation of the up- 
per Silurian strata of Herefordshire and Shropshire, the first of which 
carried down the Wenlock limestone to a depth exceeding 500 feet, 
to allow the deposition of the lower Ludlow beds and the Aymestry 
limestone ; and the second of which depressed the whole of these 
formations to a depth sufficiently great to permit the upper Ludlow 
strata to be deposited upon the surface where the Aymestry corals 
had grown. He thinks, however, from analogy, that the sinking of 
the bed of the ocean probably went on during foe whole period, ‘but 
perhaps at different rates. 
2. The attention of the author was drawn to the phenomena 
which form the subject of the second point in the communication, by _ 
the Rev. T. 'T. Lewis; -but before he enters upon their details, he - 
states, that the effects of upheaval and denudation on the upper Si- 
lurian strata of this part of England are strictly of the same order 
as those in the Bernese Jura, described by M. Thurmann*; there 
—* Essai sur les Soulevemens Jurassiques. 
