PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 431 



Thomsonite and prehnite.] 



thickness and the colors do not rise above the first order. But in sections that are 

 transverse to the elongation the double refraction is higher, giving blue of the first order. 

 In certain parts of the slide the long, coarse fibres show their rectangular, almost 

 square, cross sections. These become inclined in other places and their sections are 

 elongated, running to points at each end, and in others the elongation is almost, or 

 quite, parallel to the direction of the section. This association is illustrated by 

 figure 10, plate I. The mineral being biaxial, these rectangular basal sections 

 indicate an orthorhombic crystalline structure, and the higher basal double refraction 

 also indicates that the axial plane is perpendicular to the fibration. This position 

 of the axial plane is shown also by the appearance of both optic axes and bisec- 

 trices in sections cut parallel with the elongation, as well as by the direction of 

 retreat of the hyperbolae in sections perpendicular to the bisectrices. This is shown 

 by figure 24. 



This mineral has parallel extinction. It occurs not only as coarse 

 fibres but shares in the finer, fresher network which occupies the space 

 between the radiating coarse fibres. The more decayed older fibres 

 show high relief (or absorption) in convergent light on lowering the 

 lower nicol in contrast with that of the fine network. Micro-chemical 

 test gave lime and soda, indicating thomsonite. 



FIG. 26. MANNER 



b igure 26 shows the manner of grouping of the minute fibres of OF GROUPING 



OF PREHNITE 



l>i'<-luiite in the network between the coarser fibres. 



Interlaced between these coarser fibres are fine radiating nests of a brightly 

 polarizing mineral whose fibrous arrangement may be represented as stated by figure 

 26. So far as observable, these fibres are continually negative in elongation, but they 

 frequently are cut perpendicular to m , giving for that reason their bright colors. 

 They are, probably, in part, a finer condition of the mineral represented by figure 

 24. But as they never show, so far as observed, their highest coloration at the 

 centres of the nests, but in the spreading rays '(a), the axial plane is not perpendic- 

 ular to the fibration. This mineral is markedly distinct from thomsonite (above) 

 on lowering the lower nicol, owing to the partial decay of t*he thomsonite and not 

 because of the difference of refractive index. The thomsonite appears to be the 

 more refractive, but by the Becke process it is easily seen that the characteristic 

 white band separating two grains adjacent, of the two minerals, moves, on raising 

 the objective, toward the lighter colored mineral. Hence, the lighter colored mineral, 

 lying between the coarse thomsonite needles, has all the necessary characters to 

 show that it is prehnite, of which there is a notable amount in the rocks of this 

 neighborhood. 



There is, however, still another mineral in this slide. It is in long single fibres, 

 and has no parallel extinction, but extinction occurs at a maximum of 45. Such a 



