B. K. Emerson — Th I><,rti<hl I>'jl< ,m<l its Minerals. 277 



ization and the peculiar striation of prehnite could be clearly 

 seen. The latter mineral was, however, for the most part 

 changed into a red-brown indistinctly scaly mass, with very 

 faint polarization. The fibers of the original prehnite had 

 not been at all fused together or changed in their relation to 

 each other, and nothing was interposed among them. Other 

 nodules completely changed showed also the network of black 

 material between the needles. In specimen No. 92 of the same 

 collection with H. 15, the fibrous structure is much less dis- 

 tinct than in the former one, both are of the same dull blacfe 

 color, but under the microscope it is seen to be made up of the 

 faintly scaly red-brown material arranged in radiated 



needles exactly as before, but now showing no trace of the 

 further presence of unchanged prehnite. The brown material 

 is not to be distinguished under the microscope from that 



from the change of diabantit 

 each case the structure of the original mineral is retained. 



In other cases the change of the prehnite has taken another 

 course. In the variety from the new cutting at Cheapside de- 

 scribed on page (270), where the lustrous bars of the mineral 

 are interwoven with minute green fluor, the bars change toward 

 the side where the fissure in which they were formed opened 

 into the main vein, gradually into ;i pale sriven <ev\\- muss 

 which retains for a distance the shape of the bars and their re- 

 lation to the lluor, bul farther on is blended into a pale green 

 mass with satiny luster, which looks as it' it had been worked 

 up into a paste and dried in a thin layer upon the surface. 

 Similar masses are (ound abundantly, especially in the datolite, 

 and under the microscope contain still fragments of fresh, un- 

 changed prehnite. The mineral itself is under the microscope 

 seen to be made up of loosely aggregated scales and to show 

 a bright green color and a dichroism. both like that of diaban- 

 tite. It is apparently hexagonal, many Males remaining black 

 during a complete revolution between Nicols. Its lighter color 

 seems to be the result of its different aggregation, and the 

 powder of both is of the same light green. Prehnite can thus 

 change into a mineral very similar to if not identical with dia- 

 bantite (enough pure material for an analysis could not be ob- 

 tained), ami both the prehnite and the common scaly-radiated 

 diabaiibte change into red brown id most amorphous materials 

 which cannot be diatin ically. 



I have assumed on what seems good grounds that the 

 prehnite so often was formed 

 at the same time with the latter, and it often shows a zonal 

 o the prehnite which goes far to show that this 

 and in the specimens the two cases* can be easily dis- 



