266 



LYELL'S ELEMENTS OF GEOLOGY. 



Upper Silurian Rocks, and their Fossils. 



Among the fossil shells are species of leptsena, orthis, terebra- 

 tula, avicula, trochus, orthoceras, bellerophon, and others.* 



Fig. 275. 



Tcrelratula Wilsoni, Sow. Ludlow formation. 



Several species also of trilobite, an extinct species of crusta- 

 cean, characteristic of the Silurian period in general, are found in 

 the lower Ludlow limestone. Those represented in the annexed 

 figures, Calymene Blumenbachii and Asaphus caudatus, are 

 common to this limestone, and to the Wenlock formation which 

 succeeds next in the descending order. 



Fig. 276. Fig. 277. 



Asaphus oaiidatus. 



Some of the Upper Ludlow sandstones are ripple-marked, 

 thus affording evidence of gradual deposition ; and the same 

 may be said of the fine argillaceous shales of the Ludlow form- 

 ation, which are of great thickness, and have been provincially 

 named " mudstones," from their tendency to dissolve into mud. 

 In these shales many zoophytes are found enveloped in an erect 

 position, having evidently become fossil on the spots where they 

 grew at the bottom of the sea. Among others, the graptolite is 

 abundant. (See p. 268.) The facility with which these upper 

 Silurian shales, when exposed to the weather, are resolved into 

 mud, proves that, notwithstanding their antiquity, they are nearly 

 in the state in which they were first thrown down at the bottom 

 of the sea. All rocks, therefore, of the transition era of Wer- 

 ner, were not originally precipitated in a semi-crystalline state as 

 was formerly pretended. (See p. 153.) 



* Murchison, Silurian System, p. 199. 



