336 Geological Society. 



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, })articularly beds of Cutenipora, main- 

 tained their original vertical direction, while others Avere 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 the whole period, but 

 perhaps at different rates. 



2. The attention of the author was drawn to the phfenomcna 

 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 Soulcvemens Jurassujues, 



