1 66 WHITMAN CROSS 



has received practically no petrographic discussion, make its descrip- 

 tion at this time appropriate, although nothing can as yet be added 

 to the general statement of occurrence given by Gilbert, which is 

 summarized below. 



Features of occurrence. — The rock forms the lower of two lacco- 

 liths, of an extent not clearly determinable, owing to surficial gravels. 

 The horizons of intrusion of the known masses are in Triassic Red 

 Beds, but a few hundred feet below Cretaceous strata. The lower 

 Cretaceous beds are upturned about the laccoHth, and Gilbert con- 

 siders it probable that the intrusions occurred either in "the closing 

 epochs of the Cretaceous or the earher half of the Eocene." The 

 outcrop of the rock to be described "is nearly continuous for three- 

 fourths of a mile from north to south and more than half a mile 

 from east to west." While it is not desired to present details of 

 occurrence here, it may be noted that Gilbert observed some fifty 

 dikes traversing the sediments of horizons above that of the lacco- 

 liths, and that many of these dikes belong to the type under discus- 

 sion. The upper and probably smaller laccoHth, separated from 

 the lower one by a few feet of limestone, is composed of very similar 

 material. 



General description. — The rock of Two Buttes submitted to 

 analysis is fine-grained, greenish-gray, with a habit sometimes 

 exhibited by minette. Its most prominent megascopic constituent 

 is biotite, occurring in ghstening brown, hexagonal plates 2"^"^ 

 or less in diameter, with a few blades i^"^ long. There are a few 

 reddish-brown phenocrysts, 2 or 3"^"^ in length, representing origi- 

 nal oKvine, now wholly replaced by serpentine and chlorite, and 

 colored by iron oxide. 



A hand lens reveals many prisms of pale-green augite, and an 

 abundance of feldspar is evidently present as interstitial material. 

 A few aggregates of feldspar grains are scattered irregularly through 

 the mass, but there are no normal phenocrysts. The lens also 

 shows many minute pores, but the rock is so fresh that these seldom 

 contain secondary minerals. The fractured surface is rough or 

 hackly. 



Microscopic characters. — Under the microscope this rock is found 

 to consist essentially of a feldspathic base holding, besides the mega- 



