342 IT. S. WILLIAMS — SILURIAN-DEVONIAN BOUNDARY 



quite distinct, in the majority of its species, from the Ludlow fauna of 

 the underlying calcareous shales and limestones, but it is made up 

 chiefl y of marine species, with some traces of land plants and of mero- 

 stomes and fishes, of types similar to those which follow. Neverthe- 

 less, a few characteristic Ludlow species tie the Tilestone fauna with the 

 typical marine Silurian. The formation was more definitely described 

 by geologists following Murchison, and now goes under the names Down- 

 tonian, Downton sandstone, and Ledbury shales. The fauna is evi- 

 dently a transition fauna, and, as far as reported, appears only in sec- 

 tions in which the following formations lack purely marine fossils. 



THE TILESTONE FAUNA RECOGNIZED IN THE UPPER ARISAIG BY J. W. SALTER 



This Tilestone fauna was first discovered on the American continent 

 by Doctor D. Honeyman, in the Upper Arisaig rocks of northern Nova 

 Scotia. The identification of the fauna with the Tilestone fauna of 

 Wales was made by J. W. Salter, then paleontologist of the Geological 

 Survey of Great Britain ; an account of it was first published in 1864. 



The Arisaig rocks were brought to notice b} r Doctor J. W. Dawson* 

 (the late Sir William Dawson) in 1849, who then interpreted them to 

 be of Silurian age. In "Acadian Geology " the}' were referred to the 

 Devonian. f Honeyman/j; having studied the fossils and compared 

 them with Murchison *s Siluria, considered them mostly equivalent to 

 the Upper Ludlow in 1859. In the following }^ear Dawson,§ after more 

 careful study and the identification of fossils by James Hall, published 

 a further description of the rocks, in which he referred the series to the 

 " upper part of the Middle Silurian, probably with a part of the Upper 

 Silurian." In the same volume the species of the fauna are described 

 by James Hall.|| 



In his descriptions of the species of this fauna James Hall considered 

 most of them new, and because of resemblance in a few cases to Clinton 

 species, they were supposed by him to represent a Clinton horizon, and 

 I observe that the species are now commonly listed as Clinton species. 

 It appears to be this interpretation to which Salter refers in the passage 

 quoted below. It appears further, from notes published later by Hone} T - 

 man, that this peculiar fauna occurred in the uppermost part of the 

 zone D of his 1864 paper, which was called E in that article.^ This 

 shows that the fauna follows, in the Arisaig section, the representative 



* Quarterly Journal Geol. Soc., vol. vi, p*. r!4T. 

 fSee Acadian Geology, 1855 edition. 



I On the fossiliferous rocks of Arisaig. Trans. Lit. and Sci. Soc. Nova Scotia, 1859. 

 \ On the Silurian and Devonian Hocks of Nova Scotia. Can. Nat. and Geol., vol. v, 18G0, p. 132. 

 || James Hall: Description of new species of fossils from the Silurian rocks of Nova Scotia. Can. 

 Nat. and (Jeol., vol. v, 1800, p. 1 II. 

 \ Trans. Nova Scotia Inst., vol. iv, p. 5.~>. 



