on the Navajo Reservation. 115 



within the boundaries of the Navajo Reservation igneous intru- 

 sions in the form of dikes, sheets, laccoliths, and volcanic necks 

 occur. Few of these exhibit the features ordinarily associated 

 with igneous masses. Many of them are vertical sheets of consoli- 

 dated agglomerate in which massive igneous rock plays a minor 

 role. A few dikes and necks studied consist of over 75 per 

 cent of conglomerate, in which fragments of sedimentary rock 

 exceed in bulk the broken bits of minette, shonkinite, and augi- 

 tite. The most common foreign ingredients are blocks of sand- 

 stone and shale through which the igneous material has made its 

 way, but bits of igneous, metamorphic and sedimentary rocks 

 from unknown deep sources are also in evidence. At Alham- 

 bra Dike, granites and gneisses are embedded in the augite 

 minette. Plastered against the diabase of Church Rock two 

 varieties of granite gneiss and fragments of chlorite schist, 

 quartzite, chalcedony and augitite were found. Within one of 

 the dikes cutting Comb Monocline garnetiferous gneiss, granite, 

 slate and limestone are found among the inclusions. The 

 igneous masses within Buell's Park hold inclusions of horn- 

 blende, gneiss, garnetiferous diorite gneiss, marble, slate, mica 

 schist, and chert. It is probable that careful search for inclu- 

 sions within the igneous masses of the Reservation would result 

 in a collection containing nearly all the varieties of pebbles 

 found in the " drift " of the garnet fields. 



Conclusion. 



Materials resembling glacial deposits on the Navajo Reserva- 

 tion occur as strata interbedded with Mesozoic sediments and 

 also as superficial drift. Their position in the stratigraphic 

 column is believed to be due to igneous agencies ; their distri- 

 bution over the present surface the result of erosion rather 

 than of Pleistocene glaciation. Most of the bowlders are 

 assigned to pre-Cambrian formations. Whether the " glaci- 

 ated" forms of the pebbles have resulted from compression in 

 ancient conglomerates, from a pre-Cambrian period of glacia 

 tion or directly from igneous activity, has not been determined. 



