ART. 28 MINERALOGY OF TRIASSIC LIMESTONE SHANNON 



21 



do not penetrate quite to the base of the datolite layer so that they 

 are probably approximately contemporaneous. 



Datolite is the most abundant mineral of the low temperature 

 filled veins. The veins are narrow, averaging about 2 to 3 cm. in 

 width, and are, throughout most of their length, filled with granu- 

 lar massive datolite of a translucent pale yellowish-green color. 

 They open out into vuggy open spaces lined with crusts of crystals 

 of datolite (pL 2). Sometimes a vein so splits as to include flat 

 pieces of limestone which are coated 

 on both sides with datolite crystals. 

 The datolite rests upon the brecciated 

 limestone conglomerate which, adja- 

 cent to the veins, is comparatively un- 

 altered. The ciystals of datolite vary 

 from pale transparent yellow green in 

 the larger to opaque and white in the 

 smaller. They reach a maximum di- 

 ameter of about 7 millimeters. 



The crystal habit of mo.^t of this 

 datolite is rather unlike that of any 

 American datolite heretofore de- 

 scribed. The crystals are thick tabu- 

 lar parallel to the front pinacoid a 

 (100) and most of them, as shown in 

 figure 2, are orthorhombic in habit. 

 Quite contrary to the usual develop- 

 ment of this mineral the positive and 

 negative clinopyramids in a majority 

 of the crystals are simultaneously de- 

 veloped. Most of the faces are not 

 plane enough to afford good signals on the goniometer, and the crys- 

 tals would be considered orthorhombic on the basis of these measure- 

 ments, the deviation of datolite from orthorhombic symmetry being 

 within their limit of error. Owing to the habit of the crystals it was 

 found most advantageous to measure some of them in the Goldschmidt 

 position; that is, with the a axis (Dana) vertical. The angle tables 

 given below are in part made in this orientation and in part in the 

 Dana orientation. The figures are all drawn in the Dana orientation 

 and the indices given are the Dana indices. A small crystal of the 

 habit shown in figure 2 gave the following measurements : 



Fi(3. 2. — Datolite. Small crystal 



SHOWING COMMON HABIT WITH 

 APPARENTLY ORTHORHOMBIC SYM- 

 METRY 



