282 FINLAY 



(/?) I in an Tvpe. — The variety of the tinguaite to be described 

 under this name is found in a dike four feet wide which is ex- 

 posed for fifty feet, extending in a north-and-south direction, 

 immediately to the west of the Piedra Iman. 



Macroscopic CJiaractcrs. — In the field the rock has a light 

 brown color. Its texture is porphyritic with many very small 

 white tabular crystals of feldspar appearing as the only dis- 

 cernible phenocrysts in a dense aphanitic ground-mass. 



Microscopic Characteristics. — Under the microscope it is found 

 to contain, besides prevailing orthoclase and some little nephe- 

 Hte, smaller amounts of aegirite, magnetite and limonite. The 

 ground-mass shows a marked fluidal arrangement. 



The orthoclase appears in two distinct forms as the result of 

 as many periods of crystallization. The phenocrysts are nearly 

 always very long and narrow tablets twinned on the carlsbad 

 law, but rectangular more isometric crystals are now and then 

 found. The feldspar is always much kaolinized. The basal 

 and clinopinacoidal cleavages show distinctly. The orthoclase 

 laths in the ground-mass are very narrow, and hardly .2 mm. 

 long. 



The nephelite presents clear surfaces of low relief not broken 

 by cleavage cracks. It is found in rudely hexagonal pheno- 

 crysts which give a uniaxial figure. (See PL XI, fig. i.) 



The ground-mass contains, besides the feldspar, rods of aegi- 

 rite, small amounts of nephelite, ragged pieces of magnetite, and 

 secondary limonite. The aegirites are broader and less regular 

 than the feldspars. By weathering they take on a fibrous ap- 

 pearance and tend to lose their bright green colors. The 

 nephelite is fresh and clear. It gives rise to the deep blue-gray 

 colors with crossed nicols where it is seen in the interstices of 

 the ground-mass. 



2. Analcite Tinguaite 



{a) Mt. Armadillos Type. — The variety of the tinguaite here 

 described is found in the important dike which runs as a great 

 wall across the country from the western slope of Mt. Arma- 

 dillos to the foot of Mt. Parrefio. It is so much more resistant 



