Wolff — Occurrence of Theralite in Central America. 271 



Aet. XXXI. — On an Occurrence of Theralite in Costa Rica, 

 Central America ; by J. E. Wolff. 



Among a series of rocks collected by Prof. P. T. Hill in 

 1895 in a reconnaissance of the Isthmus of Panama and Costa 

 Rica made in cooperation with Mr. Alexander Agassiz, was 

 one small specimen occurring as an intrusion through the Ter- 

 tiary rocks of the Atlantic slope of the Costa Pican volcanic 

 plateau. Of this specimen Mr. Hill says : " The rock is pushed 

 up through the Upper Oligocene strata [limestone] and hence 

 its age is Miocene or later." According to his diagram, it occurs 

 as a large mass cutting across the bedding of the limestone. 



The rock has the typical spotted dark gray color of the 

 Montana theralite with distinct crystals of augite, biotite and 

 rounded areas of radiating zeolites. 



In thin section the rock is composed of augite, triclinic feld- 

 spar, sanidine, nepheline, a mineral of the sodalite group, 

 olivine, biotite, magnetite, apatite, and abundant zeolites. The 

 augite occurs in imperfect prisms with both pinacoids and 

 terminal planes ; it has a pale yellowish-green color with 

 marked zonal structure and an extinction of 43° on the clino- 

 pinacoid. In a large section cut across the prism the inner 

 lighter green core has the orthopinacoidal cleavage alone devel- 

 oped, while the outer zones have the usual double prismatic 

 cleavage alone developed, showing an apparent relation between 

 the development of these cleavages and the chemical composi- 

 tion. iEgirine as an outer border or separate from the augite 

 is entirely wanting. The few imperfect crystals of olivine are 

 entirely serpentinized. The biotite occurs in hexagonal plates ; 

 the apatite and magnetite require no special description. 



The feldspar is the most striking constituent in comparison 

 with the Montana theralite, for the larger part shows multiple 

 twinning on the albite law, with rarely a pericline lamella ; even 

 apparent Carlsbad twins are found by optical study to be on 

 the albite law analogous to the well-known occurrence of albite 

 itself in the crystalline schists, and the Carlsbad law seems to 

 be wanting. By means of the Federow universal stage a num- 

 ber of these feldspar sections were brought into the zone of 

 optical symmetry and by revolution on the second horizontal 

 axis the maximum negative extinction of this zone determined 

 at between 26° and 27°* ; the feldspar is therefore a labrado- 



* The use of this instrument is especially helpful in such cases where the use 

 of the excellent methods of Michel-Levy (combination of the albite and Carlsbad 

 laws) or of Fouque (bisectrix sections) is impracticable. Prof. Fedorow has 

 rendered a service to petrographers comparable to that of G-oldschmidt, himself 

 and others to crystallography in the invention of the theodolite goniometer. 



