BAIN. J 



IGNEOUS ROCKS. 29 



iiiiinediately above Rosiclare landing there are two dikes which Mr. 

 Johannsen describes as follows : 



Megascopically the rock is much altered, and at first sight it appears to be a 

 conglomerate with inelnded fragments of shale. These shaly particles are. how- 

 ever, an alteration prodnct of mica, as is shown by thin sections nnder the 

 microscope. Microscopical examination shows the rock to be almost entirely 

 altered to calcite. Large apatites, iron oxide, chlorite, and serpentine occur. 



The two dikes are about 10 feet apart, and are each about 10 inches 

 thick. They are vertical and apparently do not mark faulting planes. 

 The limestone in contact with them shows no apparent alteration, but 

 ]iarallel with them and disposed on either side are a number of promi- 

 nent veins of calcite carrying smaller amounts of fluorite. The whole 

 system of veins and dikes has a course about N. 45° W. 

 ^ A single specimen of ordinary diabase was obtained from near here 

 in the town of Rosiclare. It was found in digging a well, but as the 

 relations are not known it possibly represents material brought down 

 the river during Glacial times, and until further data are available it 

 can hardly be considered as representing another dike. 



Downey's bluff .—In the high bluff between Rosiclare and Fairview 

 landings is one of the most interesting of the occurrences. The rock 

 is found here both in a well-defined dike, cutting the point of the hill 

 Avith a course N, 31° 30' W., and in a thin sheet intruded between 

 beds of the limestone lying above the Rosiclare sandstone member. 

 The dike itself is 4 to 5 feet wide. The main sheet running off from 

 it is 6 inches thick, and about 3 inches above this is a second sheet 

 barely 1 inch thick. The two are connected by very thin stringers 

 of igneous material, indicating apparently a condition of high fluid- 

 ity. The dike does not mark a faulting plane, though on the north- 

 west side of the hill the sheet is cut off by an east-west fault of 70 

 feet, with downthroAv to the north. The dike has been traced to 

 the north about 1,000 feet to a spring, beyond which it is either 

 cut off by the fault or fails to outcrop because of the presence of 

 alluvium. 



Despite the penetration of the rock into the thinnest crevices of 

 the limestone, there is no nuicroscopic evidence of the alteration of 

 the latter by contact metamorphism. Mr. Johannsen examined a 

 number of specimens of the contact rocks, and notes that " there is 

 nothing to indicate a change caused by the intrusion." His petro- 

 graphic description of the rock, which he determines as a lampro- 

 phyre, is as follows : 



Megascopically this is a very dark porphyritic rock, consisting of a fine- 

 grained groundmass full of phenocrysts of dark mica and pyroxene. The mica 

 generally oecnrs in basal sections of about 1 mm., though occasionally crystals 

 of over a centimeter occur. The crystals of pyroxene are of small dimensions. 



