proceedings: geological society .185 



THE GEOLOGICAL SOCIETY OF WASHINGTON 



The 290th meeting was held in the lecture room of the Cos'mos Club, 

 on January 27, 1915. 



INFORMAL communications 



R. S. Bassler: Unconformities in limestone. At a previous meeting 

 remarks had been made to the effect that the Silurian limestones of a 

 certain area passed without break into the Devonian. This suggested to 

 the speaker the idea of citing examples where this was apparently so, but 

 where in reahty an unconformity of great extent was present. A fragment 

 of limestone, only a few inches thick, from Louisville, Kentucky, crowded 

 with Middle Devonian fossils on one side and Middle Silurian fossils 

 on the other, was exhibited to show that such unconformities exist in 

 what is now the same layer. Hand specimens of Ordovician limestone 

 preserving the unconformity between the lower Chazy and Black River 

 formations and taken from the same iDed were also exhibited. In fresh 

 fractures the unconformity can hardly be detected, but upon weath- 

 ered surfaces it is shown as a distinct wavy line. 



Discussion: G. F, Loughlin remarked that Bassler's examples 

 were duplicated in certain occurrences in the Tintic District. T. Way- 

 land Vaughan spoke of the contact of Cretaceous and Eocene lime- 

 stones along the Frio River in Texas. The two formations were so 

 similar that it was only by fossils that the hiatus could be detected, but 

 after the necessity for it had been discovered a thin band of pebbles was 

 found at the contact. 



REGULAR PROGRAM 



E. S. Bastin and J. M. Hill: Ore deposits of Gilpin County, Colorado. 

 (Published in full on pp. 160-164 of this issue of the Journal.) 



Discussion: F. L. Ransome inquired regarding the genetic connec- 

 tion between the different classes of ore-bodies, and if there were any 

 indication of the galena giving out with depth. Bastin replied that the 

 genetic connection was not entirely clear in all cases, but the uranium 

 ores are probably a local phase of the gold-silver ores. The tungsten 

 ores seem to be so localized as to indicate that an unusual phase of mag- 

 matic differentiation had been accompanied by an unusual phase of ore- 

 deposition. With regard to the second query — the deposition of ga- 

 lena-sphalerite ores followed a second period of fracturing and occupied 

 the spaces thus opened, and also partly replaced primary ores. The 

 natural consequence is that at ends of veins and bottoms of veins the 

 earlier minerals predominate. It is not certain, however, that some 

 galena-sphalerite veins may not persist. F. L. Hess said that his ex- 

 perience in a portion of the district indicated that tungsten ores, where 

 found, did not completely take the place of other ores. Bastin agreed. 

 J. B. Umpleby inquired regarding the sequence in time and space of the 

 pyritic veins and the galena-sphalerite veins. Bastin replied that this 

 brought up an interesting but complex problem. He referred to J. 



