122 TRANSACTIONS OF THE [mAK. 28, 



SEA-COAST DEFENSE. 



(Illustrated with lantern views.) 



March 28, 1887. 



STATED MEETING. 



The President, Dit. J. S. Newberry, in the chair. 



Thirty persons present. 



The following paper was read by title : 



THE genera and SPECIES OF NORTH-AMERICAN CARBONI- 

 FEROUS trilobites, by Lieut. A. W. Vogdes. 



(Published in the Annals, vol. IV.) 



Mr. George F. Kunz read an informal paper on 



CRYSTALS OF HOLLOW QUARTZ, FROM ARIZONA. 



(Illustrated with specimens and drawings.) 



These crystals are found about three miles southwest of the 

 town of Pinal, in Pinal Co., Arizona, occurring in a sandstone 

 rock, which has been traced for about one mile. It is found to 

 be penetrated in many places by spherules of obsidian, one-eighth 

 to three inches across, which cover the ground like a shower of 

 large hailstones. 



These quartzes are of exceptional interest from the fact that 

 they are mere walls surrounding hollow spaces much larger than 

 the area of the wall itself (see Figures 1, 5, and 8). In some in- 

 stances the crystals seem to have formed from some radial point, 

 and numbers of the detached crystals show this very strikingly, 

 especially Figures 1, 5, and 7. At the point of attachment on a 

 number of crystals there appear small nuclei of chalcedony, and 

 in some instances they radiate from this chalcedonic centre so 

 as to form veritable quartz flowers, as in Figure 2. Quite often 

 they are single individuals hollowed out, leaving only the smal- 

 lest edges of what ought to be a pyramid, and the interior of the 

 crystal being unfilled or hollow to the base in a few instances. 



