34 H. S. Washington — New Leucite Rock. 



for rock has been discovered. A more precise definition 

 will be given on a later page. 



The rock was first brought to my attention in October, 

 1919, by Baron Dr. G. A. Blanc, of the University of 

 Borne, and Ing. F. Jourdain, who had discovered it dur- 

 ing the preceding summer on the west slope of the 

 volcano of Rocca Monfina, north of Naples. These 

 gentlemen stated that the rock formed apparently a flow, 

 not a mass of tuff, more than a hundred meters long and 

 of considerable thickness. As I was on the point of 

 leaving Rome for Washington, I had no opportunity to 

 visit the locality, but Signori Blanc and Jourdain very 

 generously gave the specimen shown me, and also their 

 permission to publish the results of my study of it. 

 They also kindly promised to send me more material and 

 fuller notes on the occurrence of the rock. For this 

 courtesy and generosity it is a pleasure to express my 

 most sincere thanks. The present note is to be supple- 

 mented by a more complete description and by additional 

 analyses, when I shall have received more material and 

 fuller notes on the occurrence. "What remains of the 

 specimen has been deposited in the Petrographic Refer- 

 ence Collection of the U. S. Geological Survey. 



Megascopic characters. — My specimen of the rock 

 (which weighed only about 80 grams), is almost white, 

 though with some yellowish stains, because of superficial 

 weathering, but the interior is fresh, as the analysis and 

 the microscope show. One side is formed by a smooth, 

 slightly warped surface, such as is common with blocks 

 that have resulted from cracking during the solidification 

 of very viscous lavas. 



It is rather coarsely granular, composed almost wholly 

 of spheroidal crystals of leucite, of fairly uniform size, 

 from 3 to 5 mm. in diameter. These are water-clear, 

 with conchoidal fracture and the somewhat greasy luster 

 that is characteristic of many Italian leucites. The 

 grains show few, if any, crystal faces, as if their growth 

 had been more or less limited by mutual interference, so 

 that they are equantly anhedral. In irregular spaces 

 between them is a small amount of an aphanitic, light 

 gray substance, of dull luster, which was at first taken 

 for kaolinitic decomposition products, but which the 

 microscope shows to be glass. This glass acts as a 

 cement between the leucite crystals, which are, however, 



