198 Weed and Pirsson — Bearpaw Mountains of Montana. 



whole the agreement is very satisfactory ; the Arkansas type 

 does not contain so much sodahte and it does not in conse- 

 quence contain so much alkali. An analysis of the Brazilian 

 rock containing pseudo-leucites described by Graeff"^ and 

 Hussakjf we have not been able to find. 



In examining the face of the dike carefully the pseudo- 

 leucite phenocrysts are seen to present the distribution indi- 

 cated in the accompanying diagram (fig. 4). In the main 

 body of the dike these crystals occur somewhat abundantly 

 scattered through the rock, but about 3 inches from the actual 

 contact there is a band running parallel to the contact plane 

 ,,,. ,, and from 4 to 6 inches wide, in which 



;i|3^i 5/feei* ^-j^^ ^^^j^ shows no leucite phenocrysts. 



Outside of these bands, which occur on 

 both sides of the dike, the rock for 2 

 to 3 inches from the contact is quite 

 like that of the main body of the dike, 

 and the leucite phenocrysts occur per- 

 haps somewhat more abundantly, even 

 at the very contact surface itself. The 

 rock forming the dense band just noted 

 is quite like that of the main mass of 

 the dike, except for the absence of 

 the pseudo-leucites. The thin section 

 shows sodalites of very idiomorphic 

 J^ form, and the groundmass the same 

 Fig. 4. Cross-section of mossy appearance from the innumera- 

 contact of tinguaite dike and ble fine needles of segirite. The con- 

 ZZi^^e^^. taet of the dike with, the shales shows 

 that the pseudo-leucites and sodalites 

 occur up to the very plane of contact and of the same size as 

 in the main portion of the dike. The groundmass in which 

 they lie has, however, at the contact a very different character 

 from the main type, and this would seem to indicate clearly 

 that these phenocrysts were of intratelluric formation and had 

 been brought up in the ascending magma. The groundmass 

 appears with low powers of a uniform olive-green color, and 

 does not act upon polarized light. With the highest powers 

 it appears full of tiny shreds, fibers, and scales of almost sub- 

 microscopic fineness; they do not appear to react on light, and 

 the base seems most probably here composed of glass and filled 

 with globulitic bodies recalling the much disputed microf el- 

 site. Scattered through this are many minute sodalites, with 

 numbers of the large-sized ones. The pseudo-leucite seen in 

 the section shows so well the radial arrangement of the feld- 



* Jahrb. fiir Min. 1887, vol. ii, p. 257. 

 f Ibid 1890, vol. i, p. 167. 



