28 FLUORSPAR DEPOSITS OF SOUTHERN ILLINOIS. I m'LL. 255. 



The individual occurrences are described below. £jroui)ed l)v locali- 

 ties, tlie petro<2;raphic notes 'oeing- by Mr. Albert Johannsen. of the 

 United States (leological Survey. He makes the following general 

 observations on the rocks, based on an examination of material from 

 both the Kentucky and Illinois occurrences : 



So far as they can be cleterutiiied front the altered state of most of the rocks, 

 they fall into two groups, niica-peridotites and lamprophyres, the majority of 

 the specimens belonging to the latter. In all the specimens (of the lampro- 

 phyre) the original minerals are so much altered to caleite that subdivision Is 

 impossible, for in no ease is there any of the feldspar remaining. It is possible 

 that there was originally no feldspar, although the disposition of the particles 

 and replacement products seems to point in some of the thin sections to a grovuid- 

 mass originally in lath-shaped crystals. 



There is a sharp distinction between the mica-peridotites and the lampro- 

 phyres, however, in that in the former, olivine, eitlier jiartially or entirely ser- 

 pentinized, is clearly sho\\'n, while in the latter there is no indication of its ever 

 having occurred ; again, in the latter there is a remarkable and unusual occur- 

 rence of apatite. This occuis in nearly all of the lamprophyre slides in large, 

 perfectly fresh, short, thick prisnis, their unchanged condition making a strik- 

 ing contrast with the state of alteration of the remainder of the sections. The 

 jtresence of phlogopite in botli groups, rather than a more usual variety of mica, 

 connects them in a manner. 



Orrs landhig. — About a quarter of a mile below" Orr's landing 

 (SW. i NE. I sec. 33, T. 12 S., K. 8 E.) the St. Louis limestone is 

 cut near the water's edge by a vertical dike about 10 inches thick. 

 This shows, on superficial examination, very little igneous material. 

 The bulk of the dike is made \\\) of clastic material, consisting of 

 rounded pebbles of limestone and quartzite. These attain a maxi- 

 mum diameter of 6 inches, and, while the country rock is St. Louis 

 limestone, they include lithologic types not found in that formation. 

 The pel)bles are in a matrix of dark calcareous material, in which, on 

 examination, Mr. Johannsen found much pyrite, some magnetite, 

 quartz, and large apatites. He regards it as a lamprophyre now 

 nearly entirely altered to caleite. There are 3 to 4 inches of caleite 

 parallel to the dike material and separating it from the wall, and in 

 small seams intersecting the wdiole caleite and fluorite are developed 

 together. On the east side of the dike a narrow irregular crack leads 

 oil' through the limestone for a number of feet. Along either side 

 of this crack the limestone is altered and darkened for about 2 inches. 



This sonunvhat peculiar dike seems to represent the intrusion of 

 liquid and not entirely dry magma into an open crevice partially 

 choked by gravel and waterworn material washed in from above. 

 There has been appai-ently some slight alteration of the limestone, 

 though the natuiv of this alteration has not been studied, and whether 

 the introduction of the fluoi-ine was incident to it is wholly unknown. 



Rosiclare quarry. — In the quarries in tlie Ste. (lenevieve limestone 



