The Shinumo Area. 519 



izing the mass as a whole ; sometimes this texture becomes 

 very coarse, the separate crystals of augite or plagioclase attain- 

 ing an inch in length. The second type occurs in typical peg- 

 matite dikes cutting the diabase in many places and varying in 

 width from a few inches to several feet. The minerals are 

 plagioclase and augite and the texture is usually ophitic, 

 although not always so. In some of these dikes crystals of 

 plagioclase exceeding three inches in length were observed. 



The contact facies of the diabase are frequently line-grained 

 or glassy, but never for more than a few inches from the con- 

 tact. The slides typical of this zone reveal a hyalopilitic 

 arrangement of glass with skeleton crystals of magnetite 

 between badly altered crystals of feldspar. 



For about a half-mile east and west of the Shinumo there 

 occurs in the upper part of the diabase sill along the upper 

 contact a pink holocrystalliDe rock of medium grain. The con- 

 tact of this rock with the overlying blue slates is sharp and 

 well-defined. Downward it appears to grade into the normal 

 diabase, and no definite line of contact can anywhere be 

 observed. Unfortunately the writer did not collect transition 

 specimens, but took only one specimen from the middle of the 

 pink mass. The slide from this specimen when examined 

 under the microscope showed it to be a granular rock of 

 medium texture, consisting of rather fresh crystals of ortho- 

 clase, with subordinate quartz and a somewhat altered ferro- 

 magnesian mineral which was made out to have been originally 

 a hornblende. Some of the quartz displayed a micrographic 

 arrangement within the feldspar. The rock is a typical horn- 

 blende-syenite, and is apparently an interesting example of 

 differentiation in place within the diabase sill. But a more 

 complete set of specimens across the apparent transition zone 

 must be collected before such a conclusion can definitely be 

 established. 



In the Asbestos Canyon both the lower and upper eruptive 

 contacts of the diabase are ragged and considerably injected. 

 Many small dikes penetrate the country rock from the main 

 mass. They are glassy in texture and badly altered. 



Ransome, in his report upon the "Geology of the Globe 

 Copper District, Arizona " (Ransome h, p. 80 ff.) describes a 

 diabase of post-Carboniferous age occurring in thick sills in 

 the pre-Carboniferous sedimentary rocks of that region. This 

 diabase closely resembles the Algonkian diabase described 

 above, both in mineralogical character and in the presence of 

 the ophitic balls of plagioclase and augite. The analogy is 

 made the more striking by the fact that several small masses 

 of pink hornblende-syenite are described occurring within 

 the diabase sills of the Globe district as a possible segregation 

 phenomenon within the diabasic magma. 



