610 THE UPPER SILURIAN. 



ventricose ; ventral margin near the posterior end a little sinuous, or 

 indented from the inner side. Surface smooth under an ordinary lens. 



Two specimens only of this species have been observed, both of 

 them having the same dimensions, Arisaig, coll. J. W. D. 



Tentaculites distans, var. The specimens under examination do 

 not present any important points of diflference from those of the Clinton 

 group in New York. In the Nova Scotia specimens there are numerous 

 annulations near the apex, which are not observable in the New York 

 specimens. Arisaig, coll. J. W. D. 



Note on the Character of the Metamorphic and Igneous Rocks of the 



Upper Silurian District. 



These rocks introduce us more distinctly than the previously de- 

 scribed Devonian series to those changes in sedimentary deposits 

 known to geologists by the term metamorphism, and which, whatever 

 views may be entertained as to the precise causes of such changes in 

 the case of particular rocks, may be stated in general terms to be the 

 results of heat and pressure acting in the presence of moisture at great 

 depths in the interior of the earth. In so far as the Upper Silurian 

 rocks of Nova Scotia and New Brunswick are concerned, the alteration 

 they have undergone and the unstratified masses introduced among 

 them, may be grouped under the following general statements : — 



First, Some portions consist of shaly, sandy, and calcareous deposits, 

 considerably hardened and much disturbed, yet retaining abundance 

 of fossil shells and other evidences of a marine origin. In these, with 

 the exception of mere hardening, the changes which have been in- 

 duced on the sediments are limited to the following: — (1.) The pro- 

 duction of slaty structure, by which the rocks are rendered fissile in 

 planes different from those of their true bedding. Tliis is termed 

 slaty structure as distinguished from mere shaly lamination, and is 

 believed to be due principally to mechanical pressure. (2.) In con- 

 nexion with this, much distortion of the fossils, so that their propor- 

 tions and general forms cannot be relied on as specific distinctions. 

 (3.) The production, to a greater or less extent, of crystalline struc- 

 ture ; for example, in the New Canaan beds fossils occur in rocks 

 which have to a great extent been converted into micaceous schists. 



Secondly, Other portions, originally no doubt similar to those now 

 less altered, are highly metamorphosed, and are not only hardened and 

 schistose, but have assumed a crystalline structure, so that instead of 

 the mud, sand, and similar materials of which they were originally 

 composed, we find more or less crystalline aggregates of quartz, mica, 



