296 GEOLOGY OF THE DEBTEE BASIN. 



tlif individuals <>t' stilbite and laumontite mutually interfered with each 

 other's growth, producing the even-grained mixture. 



The fissures produced by contraction during the cooling of the mass 

 would naturally furnish channels l>\ which solutions from above might 

 gain access to cavities in their courses, while excluded from others. And 

 these very fissures are now indicated l>v veins filled with thomsonite spher- 

 ules, laumontite, and bole. In the fact that the deposit in the fissure cor- 

 responds to the last layer in the cavities appears the explanation of the 

 close of the period. When changing conditions caused the formation of 

 thomsonite to succeed that of stilbite there began a deposition in the fissures 



themselves, and with the filling of the fissures the access of solutions to the 



cavities was cut off. As soon as this stoppage became complete the spaces 

 remaining in the partly Idled cavities were placed upon an equal footing 



with the other pores, and further deposition of zeolites could take place 

 only in all alike through the slow penetration of the rock by the solutions. 



'This is found to agree with the occurrence >>( species of the second period 

 without reference t<> the presence of an earlier deposit. 



It has been brought out earlier in this chapter that the sheet of basall 

 now yielding these zeolites was quickly followed by a second flow, and 

 also that both sheets were probablj covered by strata of the Denver 

 formation, after tin' manner of the earlier basalts of the mountain. If 

 these lava flows were poured out upon a sea-bottom we can suppose the 



resulting conditions to have Keen exceedingly favorable to the formation of 

 zeolites such as are actually found. The porous zone between the tlows 

 would afford a channel tor the comparatively ready circulation oJ sea 



water which would become highly heated from the half solidified lava 



To these exceptional circumstances may he plausibly attributed the 



COUStanl presence of ferric oxide, chemically combined, in all the silicate 



minerals of the first period. 



AUGITE-SYENITE. 



This rock is known only in a few small, irregular masses in gneiss, 

 at a point west of the head of the north fork of Turkey Creek, and about 

 I. 1 , miles \V. 20° S. of Morrison. It occurs in apparently irregular-shaped 



masses, forming small knolls, whose outlines can not he accurately deter- 

 mined owing ti> the heavily wooded ground which conceals. the outcrops. 



