0. C. Farrington — Datolite from Guanajuato. 285 



Art. XXXVII. — Datolite from Guanajuato; by Oliver C. 



Farrington. 



While .examining, about two years ago, the mineral collec- 

 tion of Sr. Ponciano Aguilar, the well-known mining engineer 

 of Guanajuato, Mexico, my attention was attracted to some 

 small transparent crystals accompanying a specimen of calcite 

 from the San Carlos mine of Guanajuato. 



As I could not at the time identify the crystals, Senor 

 Aguilar kindly presented me with the specimen that I might 

 give them further study. A subsequent determination of the 

 blowpipe characters showed the mineral to be datolite, a 

 species which has not so far as 1 know been previously reported 

 from this locality. Examination of the crystal form showed 

 the habit and development of the crystals to be unique and 

 apparently peculiar to the locality. They were accordingly 

 made the subject of the following study. 



The calcite crystals with which the datolite is associated 

 have the form of — J rhombohedrons which are coated with 

 drusy calcite. Small quartz crystals about 5 ram in length and 

 made up of the usual prism terminated by the + and — rhom- 

 bohedron also accompany them. Regarding the nature of the 

 occurrence of the specimen, Senor Aguilar was unable to give 

 any further information than that it was from the mine already 

 mentioned. The rocks of the region are, however, well known 

 to be altered andesites. 



The datolite crystals are transparent, colorless, of vitreous 

 luster and small size, the largest being not over 4 mm in length 

 by l mm in thickness. Upon them a total number of 17 forms 

 was identified, as follows, the symbols being in accordance 

 with those chosen by Dana : 



a(100, i-l) cc(102, +5) k(115, |) 



5(010, a) 2(302, (J-*) /x(114. i) 



e(320, i-J) £(013, J-i) A(113, |) 



w(ll0, I) 0(012,*-*) e(112, i) 



mJOU, 1-1) villi, 1) 



/(130, i-3) 



The faces are fairly sharp and bright but in the zones of 

 prisms and hemi-pyramids merge into one another so that it is 

 difficult to distinguish them. The crystals all have* about the 

 same number of faces, but differ in the faces developed. The 

 prisms have about the same development on all the crystals. 

 Thus e, 320, is scarcely more than a line; m, 110, and o, 120, 



