230 Bulletin of Laboratories of Denison University [Voi. xii 



Keith. Contains quaitz and orthoclase, with suboidinate sericite, 

 chlorite and biotite. \V. F. Hillehiand, analyst. Rulletin No. r6S, 

 U. S. Geological Survey, p. 52. 



III. Spherulitic rhyolite — Sam Christian gold mine, Montgomery county, 

 North Carolina. Described by J. S. Diller, American fotiinal of 

 Sd'eiite, Vol. VII (1899, 4th ser.), p. 341. \V. F. Hillebrand, analyst. 

 Bulktin iVn. f68, U. S. Geological Survey, p. 53. 



IV. Quartz porphyry. -Yogo Rock, sheet at head of Belt and Running 

 Wolf Creeks, Little Belt Mountains, Montana. Described by Weed 

 and Pirrson. TivenUelh Annual Report, Part IH, U. S. Geological 

 Survey, pp. 520 ff. W. F. Hildebrand, analyst. Rullftin No. r6S, 

 U. S. Geological .Survey, Vol. Ill, p. 125. 



V. Quartz porphyry. — Six miles east of Ironton, Missouri. Described 

 by E. Haworth, Annual Report, Missouri Geological Survey, Vol. 

 VIII, 1894, p. 181. Melville, analyst. 



This analysis is compared in the table with a recent, more 

 detailed one (II), of a quartz porphyry occurring in the north- 

 western part of the state, and with a spherulitic rhyolite (III) 

 found cast of the Charlotte locality in Montgomery county ; and 

 with analyses IV and V of well known quartz porphyries occur- 

 ring in other parts of the United States. A perusal of the fig- 

 ures given in the table will make clear the general similarity of the 

 rocks, notwithstanding the rather striking differences indicated 

 in some of the constituents. 



WEATHEKING. 



In some exposures of the leopardite the weathered surface 

 of the rock, which is still hard and firm, presents a lusterless, 

 dead, chalk like whiteness, the black spots of which are more 

 or less bleached, changed from black to a reddish-brown in 

 color. This alteration is brought out fairly well in F'ig. 4, 

 which is a photograph of a hand specimen of the partially weath- 

 ered rock, reproduced one-half natural size. Bleaching of the 

 spots is more emphasized along the top of the specimen, shown 

 in the figure (4) in the contrasted lighter color of these spots to 

 others in the same figure When Fig. 4 is compared with those 

 of the fresh rock, Figs, i, 2 and 3, it is noticeable that all the 

 spots in it have undergone some leaching, as indicated in their 

 color being less intense or deep than for those in the fresh speci- 

 mens of the rock. 



