TYPICAL LOWER SILURIAN FOSSILS. 367 



The lists of fossils from the Lower Silurian thus far given arc inten- 

 tionally limited mainly to certain abundant and representative classes. 

 In addition to these a few species can be enumerated as belonging to all 

 or nearly all of the Lower Silurian rocks: 



Streptelasma corniculum, Hall. Bellerophon bilobatus, Sowerby. 



Orthis testudinaria, Dalman (vane- Murchisonia gracilis, Hall. 

 ties). ■ Murchisonia milleri, Hall. 



The Devonian. 



Localities. — Locks of this age are found in Mower county at Austin and 

 Leroy and eastward from Grand Meadow; and in Fillmore county at 

 Spring Valley and in the southwestern corner in several exposures of a 

 porous crumbling rock. This formation doubtless underlies all the ter- 

 ritory between the points named. 



Structural Characters. — In its lower layers the Devonian is a medium 

 grained siliceous limestone. Near Austin it is gray in color. It has a 

 very harsh feel. It stains readily when exposed, either at the surface or 

 along its joints, to atmosphere and moisture, becoming dirty brown in 

 color or, as at Spring Valley, of a yellowish tint. It is usually quite 

 massive, breaking into irregular blocks when quarried. In some locali- 

 ties it is quite porous, due in part to the removal of some of its mineral 

 matter and in part to the presence of large numbers of casts of some half 

 dozen fossil species.* 



Lithologic Characters. — A medium texture, a granular condition, and 

 in places a crystalline or a semi-crystalline character prevails. So far as 

 the specimens at hand have been examined, they lack the rhombohe- 

 dral form of grain so predominant in the upper Cambrian and Lower 

 Silurian. This fact is doubtless due to the compacter condition of these 

 and the additional fact, to which the rocks everywhere bear evidence, 

 that they are far less altered than are those of the two groups named. 



Paleontology. — The fossils of the Minnesota Devonian are few and 

 poorly preserved. Casts of Atrypa reticularis, Hall, Spirifera pennata, 

 Hall, and other brachiopods not yet determined occur, together with 

 several gasteropoda. Heliophyllum halli and Cyathophyllum (sp.?) have 

 been found inthedrifl of Mower comity. Many bowlders of a Devonian 

 coralline limestone are picked up around Austin, and they, if searched, 

 would no doubt disclose several species. 



*Some details not here mentioned can be found in the Geologj of Minnesota, Final Report, vol. 

 i, 1884, pp. 303, 367. 



