30 FLUOKSRAK DEPOSITS OF SOUTHERN ILLINOIS, f bull. 255- 



Under tbo microscope the structure is holocrystalliiie porphyritic, the ground- 

 mass being nnich less in amount than the phenocrysts. The pheuoerysts are 

 irregular in outline and of vai-yiug sizes, and are, in the order of their impor- 

 tance, mica, pyroxene, apatite, iron oxide, and, possibly, perofskite. The mica 

 occurs in large crystals and is chiefly phlogopite, with biotite in lesser amount. 

 The iron mineral is titanite and, possibly, considerable titaniferous magnetite. 

 Pyrite occurs in several of the slides. The pyroxene is usually nmch altered, 

 but in section No. 4 there are remnants of a pleochroic augite. The groundmass 

 is largely altered to calcite. The secondary products are calcite, serpentine 

 (from pyroxene), leucoxeue, and quartz. 



Soward farm. — Northwest of Rosiclare (W. ^ sec. 31) is an inter- 

 esting occurrence of dike form which possibly indicates the presence 

 of igneous rock a little below the snrface. The rock itself is clastic, 

 and shows both angnlar and rounded fragments of limestone, with 

 flakes and bits of black shale cemented by a limestone matrix. Some 

 very altered ferruginous material may represent igneous rock, but 

 this is imcertain. From the mode of occiu-rence it is possible that 

 this represents a fissure filling, largely of material brought up from 

 below by the intrusion underneath. It is mentioned merely as a place 

 at which it is thought igneous rock may yet be found in situ. 



Mrx farm. — There are two dikes known in Pope County. One of 

 them is that discovered in 1902 on the Charles Mix farm, a few miles 

 above Golconda (sec. 8, T. 13 S., R. 7 E.). It cuts Chester sandstone, 

 presumably of Birdsville age, and has a course about N, 40° W. It 

 shows only in the river bluff, and apparently the intruded rock has 

 not been in any way altered. Mr. Johannsen determines the rock to 

 be a mica-peridotite, somewhat similar to that described by Diller, 

 and describes it as follows : 



The hand specimen consists of a dark, granular, mottled black and white 

 groundmass, with phenocrysts of mica and pyroxene. Under the microsco])e 

 they very closely resemble the sections of the Flannery dike. They consist 

 chiefly of phlogopite, serpentine, ilmeuite. leucoxeue, jterofskite, magnetite, and 

 a little pyroxene and olivine. The olivine is only partially altered to serpen- 

 tine. 



Golconda. — The second occurrence of dike material noted in Pope 

 County is that found a short distance west of Golconda, at the Gil- 

 bert prospect. This dike is 4 to 5 feet thick, vertical, and cuts Ches- 

 ter — presumably Birdsville — sandstone and shale without faulting 

 them. Mr. Johannsen describes the rock as follows: 



A pyroxene-lamprophyre, consisting of much calcite, chlorite, iron oxide, some 

 l)hlogopite, leucoxeue, and some serpentine after pyroxene and, doubtfully, after 

 olivine. The iron oxide is ilmeuite, and probably some titaniferous magnetite 

 occurriug in cubical crystals. A little hematite occurs. 



