280 FINLAY 



and for many miles along the eastern and western flanks of the 

 range. In some cases single dikes may be traced for one and 

 a half or two miles with a width of ten feet. 



I . Tingtiaite 



(a) Santa Rita Type. — The variety of the tinguaite here de- 

 scribed is met in a dike between the Mesa Verde and Mt. Par- 

 reno near the pass which the Santa Rita trail takes on leaving 

 the San Jose valley. 



In the hand specimen the rock is noticeably darker than the 

 other tinguaites of the district. It is a dense compact rock 

 having a deep green aphanitic ground-mass in which tablets of 

 sanidine often 2 cm. in length occur prominently as pheno- 

 crysts. Without a hand lens it is difficult to distinguish the 

 larger crystals of segirite in the dense aggregate made up of 

 the minute individuals of that mineral. Amygdaloidal cavities 

 occur which contain tiny shreds and patches of biotite, mag- 

 netite grains and the earthy aggregates that have resulted from 

 the weathering of the zeolites. 



Microscopic Description. — Besides the sanidine and aegirite 

 the microscope shows that nephelite, plagioclase, hornblende 

 and analcite are present. 



The larger phenocrysts of feldspar are in sharply automor- 

 phic Carlsbad twins bounded by (i lo) and (lOo). They attain a 

 maximum length of 3.5 mm. and are one quarter as broad. 

 (See PI. X, fig. 2.) They contain many inclusions, minute 

 prisms of aegirite zonally arranged with their long axes parallel 

 to the walls of their host, and bundles of microscopic brightly 

 polarizing needles having a high index of refraction. The feld- 

 spar crystals show peculiar effects of weathering and secondary 

 replacement. Good cleavage cracks appear rarely. Now and 

 then kaolinized cores remain, but nine tenths of their substance 

 is usually of secondary origin, largely orthoclase feldspar, 

 plagioclase and analcite. The cores of the original crystals have 

 been eaten out and in the central portions analcite is now found. 

 (See PI. XI, fig. 2.) This mineral is isotropic, and it gives no 

 interference figure. It has two sets of cleavage cracks at right 



