﻿208 W. E. Hidden — Contributions to Mineralogy. 



plane. The fact that common monazite is described as having 

 highly perfect basal cleavage suggests that the difference may 

 be due to the thorium silicate often present as an impurity as 

 shown by Penfield. It seems probable that these crystals, like 

 the Swiss turnerite, consist of the pure phosphate of the cerium 

 metals. 



The density of monazite seems to offer a test for the presence 

 of thorium, since that from Amelia C. H., Ya., with a specific 

 gravity of 5-30, contained 14'23 per cent Th0 2 ; that from 

 Portland, Conn., specific gravity 5'20-*25, contained 8'25 per 

 cent, and that from Burke Co., N. C, specific gravity 5*10, 

 contained 5*49 per cent (analyses by Penfield). The best 

 crystals of these now described have specific gravity rang- 

 ing from 5*05-*09. No analysis of these crystals has yet 

 been made, though some excellent material has been sent to 

 Mr. Penfield for this purpose. Only about twenty good crys- 

 tals were found altogether and the present indications are that 

 the pocket is exhausted. 



Quaetz Crystals with Basal Plane. — Since 1879 the 

 writer has been much interested in the so-called "basal-plane 

 quartz crystals" from North Carolina ; his experience goes to 

 prove the fact that genuine basal planes are very rare occur- 

 rences on this species. In the great majority of cases the 

 planes observed have been produced by compression or juxta- 

 position, and when carefully measured do not meet the require- 

 ments. 



From among many crystals, appearing, when superficially 

 observed, to possess this rare plane, I have selected two only, 

 which, upon examination, proved to have a natural and nor- 

 mal development of the basal pinacoid (0). Not feeling per- 

 fectly satisfied with my determination — it being made with a 

 hand goniometer — I sent the crystal showing the smoothest 

 developed basal truncation to Prof. Des Cloizeaux for exam- 

 ination, and I add here a translation of a portion of a letter 

 from him, dated May 8th, giving his results, for which I wish 

 also to express my thanks. The crystals referred to were 

 found last summer in Sharpe's township, Alexander Co., N. C. : 



" Your crystal definitely disposes of the question of the basal 

 plane on quartz, which, until now 7 , has remained doubtful. My 

 measurements prove that the base is quite perpendicular to the 

 vertical axis, which we may question of many of the so-called 

 'basal planes' on quartz. p( + -R)A al ( 0) = 128°, calc. 128° 13'. 

 This adjustment gave an elongated reflection of which the cen- 

 ter gave 128°. p( + fi)/\p, right = 94° 12', calc. 94° 15'. The 

 face 5f (— -J-i?3) is also present on this crystal. I have identified 

 this face before on crystals from Wallis, and on amethysts from 

 Brazil and Hungary." p( + H)/\b%(-jrfi3) = W° 40', calc. 147° 

 39'. 



