172 WHITMAN CROSS 



clase) and olivine on the other. That this variation is in large 

 degree dependent on geological occurrence as influencing condi- 

 tions of consolidation has been pointed out, notably by Iddings^ 

 and Washington,^ the latter while discussing the rocks of Monte 

 Catini and La Colonetta of which analyses have been given. 



The rocks of the above tables are all intrusive, except the cimi- 

 nite of La Colonetta, which is a surface flow. The latter is the only 

 one in which biotite is not an important constituent. It contains 

 numerous phenocrysts of augite and olivine, with a few of ortho- 

 clase and labradorite in a very dense groundmass, which is "a felt 

 of minute orthoclase and some labradorite laths lying in a glassy 

 base" (Washington). 



On the other hand, the selagite or mica-trachyte of Monte Catini 

 has very closely the habit of the Colorado prowersose, as I am able 

 to state through examination of a type specimen donated by Dr. 

 Washington to the Petrographic Reference Collection of the Geo- 

 logical Survey. The development of biotite and the megascopic 

 appearance of the groundmass is very nearly the same in the two 

 rocks. Microscopically, the difference in texture is more pro- 

 nounced, as in the selagite augite occurs in small crystals forming 

 a part of the groundmass, and the orthoclase tables are much more 

 sharply defined than in the Two Buttes rock. 



In marked contrast with the syenitic lamprophyre of Two Buttes 

 stands the fergusite or pseudoleucite-syenite of the Highwood Moun- 

 tains. That rock contains numerous pseudoleucite grains 5"^™ 

 or less in diameter, held in a matrix consisting chiefly of augite, 

 with small amounts of biotite, oHvine, and other accessory minerals. 



The great textural difference between the two known types of 

 prowersose is most striking. The rock of Knox County, Maine, 

 is a most pronounced porphyry, as a reference to the description 

 by Mr. Bastin, and a glance at the photographic illustration, will 

 show. There are many large alkaH feldspar phenocrysts lying in 

 a groundmass of biotite and green hornblende. 



1 J. P. Iddings, "The Origin of Igneous Rocks," Bulletin of the Philosophical 

 Society of Washington, Vol. XII (1892), pp. 176, 177. 



2 H. S. Washington, "Some Analyses of Italian Volcanic Rocks," American 

 Journal of Science, Vol. IX (1900), p. 49. 



