354 APPENDIX. 



specimens is an irregular octahedral crystal, eight inches in diame- 

 ter. The color of these masses is various shades of blue, violet, 

 or red, sometimes perfectly white or yellow; and the form most 

 commonly assumed is a cube, sometimes truncated at two or more 

 angles, or variously clustered. The external lustre of the crystals, 

 raised from alluvial soil, is feeble, but quite brilliant when taken 

 from veins and cavities in the rock. These spars from the allu- 

 vion do not appear to exist as rock debris, or fragments worn off 

 from other formations, but as original deposits. There are no 

 marks of attrition. They appear as much in place as the lime- 

 stone rocks below. It should also be recollected that this mineral 

 tract is terminated by one of the greatest and most valuable salt 

 formations in the western country — that of the Illinois Saline. 



Septaria: Ludus Helmontii. — This variety of calcareous marl is 

 found, in orbicular or flattened masses, along the eastern shores of 

 Lake Michigan, between the rivers St. Joseph's and Kalemazo. 

 Its original situation appears to be the beds of marly clay which 

 form the banks of Lake Michigan at these places, from which 

 these masses have been disengaged by the waves, and left pro- 

 miscuously among the washed and eroded debris of the shore. 

 These masses are penetrated by numerous seams and lines of cal- 

 careous spar, sometimes radiating star-like, or intersecting each 

 other irregularly. Occasionally, these seams are filled with sul- 

 phuret of zinc, and in these cases the spar, if any be present, is 

 rose-colored. 



d. Aluminous Minerals. 



1. Argillaceous Slate. 



1. Argillite, or Common Argillaceous Slate. — Along the banks of 

 the Eiver St. Louis, at the Grand Portage, &c. It occurs in a 

 vertical position, embracing veins, or subordinate beds, of grau- 

 wakke, milky quartz, chlorite slate, and silicious slate, &c. It is 

 bounded on one side by red sandstone, and on the other by an 

 extensive tract of diluvial soil. 



2. Bituminous Shale. — In detached masses, along the shores of 

 Lake Huron, between Fort Gratiot and Thunder Bay. It con- 

 tains amorphous masses of iron pyrites, of a yellow color and 

 metallic brilliancy, which soon tarnishes on exposure to the air. 



