L. V. Pirsson — Datolite from Loughboro, Ont. 101 



Lacy Mine, Longhboro, Ontario. More magnificent crystals of 

 this species have probably never been found in America and 

 they are equalled by few European specimens. They are pure 

 and transparent with a yellowish green tinge and enclose only 

 a few small crystals of chalcopyrite as impurities. At first 

 glance they resemble large topaz crystals. In size they meas- 

 ure for the largest crystals 3 X2^x2 cm . Nothing is known of 

 the mode of occurrence save what can be gathered from the 

 examination of a small amount of associated material on the 

 hand-specimens. From this, the mineral seems to have formed 

 in veins in a light colored igneous rock too much acted upon 

 by fnmarole agencies for satisfactory determination, but con- 

 taining a considerable amount of a brownish biotite. On the 

 specimens the datolite crystals are associated with quartz, cal- 

 cite and chalcopyrite. 



The material consisted of two hand specimens with groups 

 of large crystals and one detached crystal of the size mentioned. 

 In habit these crystals are nearly all alike in the development 

 of their planes, being characterized by a prismatic like develop- 

 ment in the zone of clinodomes following the orientation given 

 this species by E. S. Dana.* 



The loose crystal could be easily measured and was also the 

 most complex in the development of its planes. The follow- 

 ing forms were identified : 



a (100) i-l 



u (104) — i-l 



re (111) — 1 



& (010) i-i 



x (102) -i-l 



e(Il2)i 



c(001) O 



? (10?) l-l 



/3 (121) — 2*2 



to (110)7 



9(012) i-i 



Q (122) — 1-2 



o (120) i-2 



m x (011) 1-i 



CT(123) -£-2 



This crystal is shown in the accompanying figure. All the 

 faces except the pinacoids and clinodomes have been relatively 



exaggerated in size to exhibit them 

 better. No other forms beyond 

 these were observed on any other of 

 the crystals. The zone o\ /?, Q, U 

 was well developed and gave excel- 

 lent reflections on the goniometer. 



The crystals were not studied op- 

 tically as no material was available 

 without damage to the specimens, 

 but, as confusion has frequently occurred in the orientation of 

 this species owing to the great similarity of angles in the 

 zones of prisms and of clinodomes, the following table of 

 measured and calculated angles is appended to show the cor- 

 rectness of the orientation and the identification of the forms. 



* System of Mineralogy, New York, 1892, p. 502 et seq. 



