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|VOL. 3. 



than of detrital origin. This will become evident from the 

 description of a few typical specimens. 



One of these, gathered near the base of the cliff at the typical 

 locality, is a greenish blue rock, rather incoherent in texture, 

 made up of unassorted volcanic fragments varying in diameter 

 from half an inch to a minimum. These fragments include bits 

 of lava and crystals, among which glistening splinters of horn- 

 blende and angular fragments of feldspar may be discerned. 



No representative slides of this rock could be prepared, but 

 the various kinds of fragments were microscopically examined. 

 The lava is found to be a pyroxene andesite of the Hald's Canon 

 type. It contains phenocysts of green augite and zeolitized 

 labradorite, with pseudomorphs of green and brown iddingsite 

 that presumably represent hypersthene in a highly decomposed 

 groundmass. The hornblende, examined in cleavage fragments 

 under the microscope, is found to be an olive-green variety with 

 a fairly large extinction -angle. 



A second specimen, from the lower leaf-bearing horizon, is a 

 fine-grained, compact rock, about as hard as limestone. In color 

 it is lead-gray, obscurely mottled with reddish purple, by which 

 the leaf-impressions and certain of the bedding planes are also 

 tinted. Small fragments of hornblende, with streaks and veins 

 of a soft olive-green mineral, are also visible macroscopically. 



The microscopical examination proves that the rock is a tuff, 

 so much decomposed, however, that the original structure is 

 greatly obscured. The original minerals, occurring as angular 

 crystal fragments, are feldspar, hornblende, magnetite, and 

 apatite. The feldspar in abundant small laths shows low 

 extinction angles characteristic of andesine. The hornblende is 

 of an olive-green pleochroic variety, with an extinction-angle 

 near 15°. Apatite often occurs in rather large prisms. A mineral 

 resembling iddingsite occurs in fine-grained spherulite aggregates 

 of oval or irregular form. 



The most interesting feature of this rock, however, consists 

 in the presence of an abundant zeolitic cement. This is best 

 observed between the crossed nieols, by whose aid it is seen that 

 the interstices between the other minerals are completely filled 

 with a clear substance having a double refraction near that of 



