NATURAL SCIENCES OF PHILADELPHIA. 303 



liibits a more distinct crj^pto-crystalliiie appearance than exists in the slate. 

 Suspecting that they possibly might be of the nature of coprolites, Dr. L. had 

 desired Dr. Genth to analyze part of one. The result was carb. lime 36-5, silica 

 and silicates 59-1, oxide of iron, alumina, etc., 4-4. Though not of vertebrate 

 origin, they may perhaps have been the excrement of some huge invertebrate, 

 which, with all others of the time, are now totally obliterated. 



Dr. Leidy further remarked that it was well known that iridescent hues from 

 the surfaces of bodies, independent of thin films, were usually due to aniinute 

 striation or parallelism in the arrangement of the elements of structure. Thus 

 is produced the iridescence upon the wings of the house fly and man}' other 

 insects, that of muscular and tendinous fibres, of pearl shells, artificially ruled 

 surfaces, etc. He has repeatedly observed that the iridescence on the surface 

 of waters was due to the same cause, through myriads of vibrios and bacteria. 

 Under the circumstances he was surprised that authors continue to repeat that 

 the phenomenon of the beautiful play of colors in the precious opal has not 

 been satisfactorily explained. It is evidently due to a regular striated condi- 

 tion of the structure, readil}' observed by the microscope. Tlie stria; upon 

 brilliant facets examined in a number of opals appear to be about 6000 to the 

 inch. The striaj are probably the pores to which Brewster alludes as being 

 the cause of the coloration of the opal. The brilliancy of labradorite is also 

 due to a regular parallelism in the arrangement of elements of structure. 



Nov. 10th. 

 The President, Dr. Hays, in the Chair. 

 Forty-two members present. 



Nov. nth. 

 The President, Dr. Hays, in the Chair. 

 Twenty-five members present. 



Mr. R. P. Stevens made the following remarks on the geology and 

 mineralogy of Venezuela : 



Observations made by my party extend two hundred and fifty miles up the 

 Orinoco river from the city of Bolivar, or five hundred from its mouth, and in 

 three directions southwards one hundred and fifty to two hundred miles. In 

 the hydrographical basin of the Orinoco we have seen no other rock than 

 gneiss, gneissoid schists, granite, and other crystalline rocks. The gneiss is 

 granular and lamellar. Its minerals are magnetic iron ore, cupriferous ores, 

 argentiferous galena, and, very sparingly, gold. 



We have been able to make out two distinct systems of elevation in this 

 basin ; one running N. and S., the other W. N. W. and E. S. E. The former 

 is a low ridge of black, shining, lamellar gneiss, forming a low divide between 

 the Caroni river and the affluents of the Yuruary, or the line of demarcation 

 between the hydrographical basins of the Orinoco and the Essequibo on the 

 west. 



Upon the flanks of these rocks abut the gneiss of the Imitaca Mountains, 

 which forms the divide between these two basins on the north. 



The remaining system of elevation runs N. E. and S. W., and corresponds 

 with that of the Appalachian system of the United States. 



This system is confined, so far as our observations have seen, to the basin of 

 the Essequibo. The rocks elevated by this system are talcose, with quartz 

 veins, quartzite, porphyry, brecciated schists, and aluminous rock of a bluish 

 color, locally known as " bluestone." 



1868.] 



