304 



University of California. 



[Vol. i. 



The mineral possesses two fairly distinct habits according as it 

 crystallizes freely in empty clefts and cavities, or in a matrix of mar- 

 garite. In the former case it has the general form shown in Plate 

 17, Fig. [, less often that of Fig. 2. The small face d' of the brachy- 

 dome '041 f Plate 17, Fig. 3, was observed on only two small crys- 

 tal fragments. Crystals possessing the habit shown in Plate 17, 

 Figs. 1 and 2, are small, generally under 5 mm. in greatest diameter. 

 On the other hand, such crystals as occur embedded in margarite 

 are characterized by their greater size, and by a prevailing tabular 

 f irm, the basal planes being well developed, and striated strongly, 

 parallel with the brachy-axis. Irregularly rounded, and deeply stri- 

 ated brachydome faces are usually present on these larger crystals, 

 but such simple forms as Plate 17, Fig. 7, are by no means rare. A 

 conspicuous extension of a pair of the prism faces in a horizontal 

 direction, as in Plate 17, Fig. 6, is so constant as to be characteristic. 

 The size of these embedded crystals is often considerable, incom- 

 plete fragments measuring as much as 5 cm. in width across the 

 basal plane, while the length of the perfect crystal in the direction 

 of characteristic elongation is in some cases five or six times that 

 dimension. 



Twins are common in both classes of crystals, the twinning and 

 composition plane being the prism. A common form of twin among 

 the large embedded crystals is shown in Plate 17, Fig. 5 and the 

 characteristic feather arrangement of the basal striae is diagrammat- 

 ically indicated in Fig. 4. In Plate 17, Fig. 8, is shown one of the 

 smaller crystals, with a habit intermediate between Figs. 1 and 2, 

 twinned according to the same law. 



Lawsonite has two conspicuous cleavages, a perfect cleavage 

 parallel with the brachypinacoid \o\o\ and a sub-perfect cleavage 

 parallel with the basal pinacoid \oo\\. Sections cut parallel with 

 the macropinacoid show these cleavages as two sets of lines cutting 

 each other at an angle of 90 . Those parallel with b are bolder, 

 more abundant, and more continuous than those parallel with b, 

 which are generally fine and interrupted. There is, besides these, a 

 third and very indistinct cleavage parallel with the prism, which has 

 not been detected macroscopically, but can be seen in basal sections 

 under the microscope when the light is properly adjusted, as a series 



