PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 169 



Thalite.] 



the amygdules is sometimes a little calcite. Its hardness is about that of talc, and 

 it forms a jelly in HC1. 



The vermicular forms are transversely fibrous. They recall those of- the 

 ripidolites (helminths) and of kaolinite. In proportion as they are curled up do 

 they approach spheruliths. When the ends of the vermicular forms come together, 

 the fibres diverge, in thin section as in a spherulith. They also diverge when the 

 section happens to cut the convex surface, from one of the vermicular shapes. They 

 are fairly illustrated by figure 1, page 390, of Lacroix's Mineralogie de la France et de 

 ses colonies. The bisectrix n g is parallel to the fibres. The images in convergent 

 light vary. The optic angle (2V) appears to be small. 



Therefore it presents essential differences from the kaolins, the chlorites and 

 from magnetite. Its bisictrix is n f parallel to the fibres in place of being n v perpen- 

 dicular to a cleavage sharing in the zone of the filaments. 



Specific gravity is 2.20. 



Chemical analysis was made of this substance by Mr. L. B. Pease, with the following result: 



SiO 2 - . 42.38 



A1 2 0, 7.37 



Fe 2 3 2.65 



MgO 23.29 



CaO 5.52 



K 2 O 0.19 



Na 2 O 0.36 



H,O 18.18* 



Total, 99.94 



Owen's description may be summarized thus: A hydrated silicate 

 of magnesia, combined with a new earth intermediate between magnesia and 

 manganese: Sp.gr. 2.548; not found crystallized, composed as follows: 



Water, 18.0 



Silica, 42.0 



Magnesia, 20.5 



New earth, not taken up by sal ammoniac, 10. 12 



Alumina, - 4.6 



Peroxide of iron, 1.5 



Potash, - 0.8 



Manganese, trace. 



" Leaving out of account the supposed new earth, the chemical composition 

 comes nearest to saponite and soapstone." 



A similar green mineral was found by Dr. Shumard three miles above Kettle 

 river, while Owen reports his thalite "from the vicinity of Baptism river." 



By reason of the foregoing distinctions, this mineral seems worthy of specific 

 independence. N. H. w. 



*At 100 10.38. 



