calkins. I Petrography of the John Bay Basin. 125 



differ considerably from bastite in their optical properties. By 

 ordinary transmitted light, the mineral is olive-green. The 

 pleochroism is fairly strong', with the greatest absorption parallel 

 to the length. Between crossed nicols the extinction is seen to be 

 straight, and the elongation positive. The double refraction, 

 which, because of the fibrous structure, could not be measured 

 exactly, is in excess of .020, and gives rise to bright colors of 

 the second order. 



Comparing these properties with those of iddingsite, we find 

 in most respects a fairly close resemblance, although the lamellar 

 structure described by Lawson does not appear to be present. 

 It seems highly probable that the mineral is practically identical 

 with iddingsite. 



It is worth noting in this connection that iddingsite has 

 always been supposed to originate from the decomposition of 

 olivine. Fibrous pseudomorphs in volcanic rocks, derived from 

 rhombic pyroxene, have been observed by Gregory* and by 

 Bonneyt. The double refraction of the mineral described by 

 Bonney is weak, while Gregory does not allude to this point. 

 The writer has found no description that agrees completely with 

 the one given here. 



The structure of the groundmass is a feature of great 

 interest. In ordinary light it appears to be of the common 

 andesitic type, being made up of feldspar laths with grains of 

 pyroxene and magnetite in a colorless, transparent base. Between 

 crossed nicols, however, this base is seen at once to be a crystal- 

 line substance, in the form of irregular, interlocking grains, each 

 of which incloses poikilitically a dozen or more crystals of the 

 other constituents. Its properties, so far as determined, are as 

 follows: The refractive index, as determined by the Becke 

 method, is higher than that of the balsam, and lower than that of 

 the plagioclase. The double refraction is slightly lower than for 

 the plagioclase. Owing to the small area of the clear spaces, it 

 was impossible to obtain a satisfactory interference figure, and 

 no cleavage was detected. Twinning does not occur. Treatment 

 with hydrochloric acid does not seem to produce any effect. The 



*U. S. Geol. Survey, Bull. 165, 1900, p. 170. 

 tGeol. Mag., 1885, Vol. 22, p. 70. 



