520 Noble — Geology of the Grand Canyon, Arizona. 



Contact metamorphism. — Since the diabase sill occupies 

 relatively different horizons in the Unkar strata upon the 

 Shinumo and in the Asbestos Canyon, and since in the former 

 locality the strata between which the sill is intruded lie in 

 undisturbed sedimentary contact in the latter locality, and 

 vice versa, the study of the respective contact effects upon the 

 invaded strata could be made with great facility. 



The contact effect upon the shales which lie above and 

 below the diabase along the Shinumo has already been 

 described in the detailed section of the Unkar : there the 

 shales were shown to be altered to jaspers by baking and silici- 

 fication. It was shown that the intensity of metamorphic 

 action was much greater below the sill than above, extending 

 through 300 feet of strata below the lower contact and through 

 only 100 feet above the upper contact. In the Asbestos 

 Canyon these rocks lie in undisturbed sedimentary contact 

 and are there unaltered red shales. 



The contact effect upon the limestones can be studied in the 

 Asbestos Canyon, where the diabase sill lies intruded within 

 them. Immediately below the lower contact of the diabase, 

 which is sharp and well defined, is a thin layer of green ser- 

 pentine. Below lie layers of pure crystalline limestone (dolo- 

 mite) alternating with layers of the same character containing 

 bands and nodules of serpentine. Within one of the layers 

 containing the bands and nodules of serpentine are cross- 

 fiber veins of golden-yellow chrysotile asbestos which are 

 parallel in general trend to the bedding of the limestone. 

 These limestones are the layers at the base of division c of the 

 second member of the Unkar. They overlie the red shales of 

 division b, which are here baked to blue slates. 



Section below diabase contact (reading upward from the base) 

 Dense, purple, calcareous slate (b, 24 of second member 



of Unkar) „ . 9' 



Soft, blue slate (b, 25) . „ 3' 



Nodular, cherty limestone (c, 1, 2) 4' 



Banded, crystalline limestone with bands and nodules of 



serpentine (c, 3, 4,) . . . 10' 



Serpentinous, nodular and banded layer carrying veins 



of asbestos 1' 



White, crystalline limestone with bands and nodules of 



serpentine . - 2' 



Pure, white, crystalline limestone ] ' 6" 



Layer of green serpentine _.__ 2' 



Diabase. 



Above the upper contact of the diabase the limestones con- 

 tain several alternating layers of green serpentine and narrow 



