212 



J. H. Pratt — Mineralogical Notes. 



uplifting recorded in the old levels must have taken place at 

 least since the beginning of the Pleistocene. In other words 

 they are comparatively modern in geologic time — some of 

 them absolutely recent, 



It is not maintained in this paper that these epochs of re- 

 gional elevation were continuous and uninterrupted, or unac- 

 companied by pauses or even alternating epochs of subsidence, 

 but the general progress of the island has been periodically 

 upward and if there were epochs of subsidence they are difficult 

 to distinguish and were of short duration and insignificant in 

 comparison with the great uplifting movement that has gene- 

 rally progressed. 



Art. XXIX. — Mineralogical Notes on Cerussite, Calamine 

 and Zircon ; by J. H. Pratt. 



1. Cerussite. — The mineral occurs at the Judge Mine, 

 Black Hawk, Meagher Co., Montana, and was collected, through 

 the kindness of Mr. Davis, foreman of the mine, by Mr. L. V. 

 Pirsson, during the summer of 1893 while engaged in field 

 work on the IT. S. Geological Survey. 



It is associated with several ores of lead resulting from the 

 decomposition of galena, the main constituent of the ore body. 

 The mineral is found in pockets in the form of rough, masses, 

 at times as large as one's fist, showing an occasional broad crys- 

 tal face. These fragments are remarkably clear and free from 

 foreign inclusions. Attached to these are small bright crystals, 

 both simple and twinned. Their faces are generally smooth, 

 giving very good reflections of the signal when measured on 

 the reflecting goniometer. 



Specimens of the simple crystals which are represented by 

 fio;. 1, were found so attached to the rest of the mineral that 



the faces on both ends of the lateral axes were developed and 

 could be measured. The forms observed on them are : 



