38 HAEHL AND ARNOLD — THE MIOCENE DIABASE. [Feb. 5, 



House, the road cuts across the contact of the diabase with the lower 

 Miocene sediments in a small canyon, so that a good cross section 

 is exposed (see Fig. 2). The sediments here dip at an angle of 

 sixty degrees south, twenty degrees west, and the diabase, which 

 is of the coarse variety and rather badly weathered, follows the bed- 

 ding planes. Within the diabase, running exactly parallel to the 

 contact and dipping with the sediments, are a number of coarsely 

 crystalline secondary dikes, varying in width from one inch to six 

 inches and standing out hard and fresh in the darker decomposed 

 diabase. Figure 2 is a photograph of this dike. The secondary 

 dikes may be seen to the left of the man in the figure. 



Macroscopically this rock is medium grained, light colored and 

 with a rather mottled appearance, due to the uneven distribution of 

 the more basic constituents. Augite, plagioclase and magnetite are 

 the chief components. The augites are large and tend to segregate 

 in spots, often with a poikilitic structure; the feldspars being in- 

 cluded in the augites and giving a mottled appearance to the rock. 

 The feldspars are long and narrow, somewhat kaolinized and show 

 banding. Clear patches of analcite are frequently included in 

 them. Magnetite is very plentiful in long irregular blades which 

 stand out prominently and often reach a length of half an inch. 



A few small cavities in the rocks are filled with a mass of rather 

 flexible, fine, acicular crystals matted together indiscriminately. 

 The crystals are usually light colored, although a few are discolored, 

 evidently by weathering products. Such small amounts were ob- 

 tained that it was impossible to determine them accurately. Before 

 the blowpipe they are infusible and they are not acted upon by 

 acids. 



The basaltic fades. — It is thought best to treat all the fine- 

 grained dark varieties which make up the remaining portion of the 

 area under the head of basaltic facies. These in turn could be read- 

 ily separated into at least two general types, differing in the coarse- 

 ness of their crystallization and weathering products, but such a 

 classification would be tedious and will not be attempted. It is 

 necessary to state, however, that those portions of the area which 

 are made up of smaller dikes are almost universally of a coarser 

 texture than the larger masses and exhibit spheroidal weathering in 

 a very striking way. Figure 19 illustrates a typical example of the 

 spheroidal weathering of the medium grained diabase. The basaltic 

 facies differ from the diabasic facies in that they are dark, show- 



