HANDBOOK FOR THE DEPARTMENT OF GEOLOGY. 571 



Substance, which chemical analysis shows to be also a mixture of quartz- 

 ose and feidspathic material. The porphyritic quartzes show frequently 

 a marked corrosive action from the molten magma, the mineral haviug 

 again been partially dissolved after its first crystallization. (Fig*. 3, 

 PI. cxx.) This difference in structure in rocks of the same chemical 

 composition is believed to be due wholly to the different circumstances 

 under which the two rocks have solidified from a molten magma. The 

 structure of the groundmass is not always felsitic, but may vary from 

 a glass, as in the pitchstones of Meissen, Isle of Arran, and the Lake 

 Lugano region, through spherulitic, micropegmatitic, and porphyritic 

 to perfectly microcrystalline forms as in the microgranites. This differ- 

 ence in structure may be best understood by reference to transparencies 

 Nos. 39075 and 39078 in the windows and to PI. cxx, which show the 

 microscopic structure of (1) granite from Sullivan, Hancock County, 

 Maine, (2) micropegmatite from Mount Desert, Maine (specimen 70041), 

 and (3) a quartz porphyry from Fairfield, Pennsylvania. 



Marked lluidal structure is common as shown by the specimens in 

 the structural series ISTos. 35946 and 35959, and also in specimens 

 35959 and 26570, from Massachusetts and Minnesota. (See PI. cxxvu.) 



Colors. — The colors of the groundmass, as above noted, vary througli 

 reddish, brownish gray to black and sometimes yellowish or green. 

 The porphyritic feldspars vary from red, pink, and yellow to snow- 

 white and often present a beautiful contrast with the groundmass, form- 

 ing a desirable stone for ornamental purposes. (Specimens 36244 and 

 36245, from Elfdalen, Sweden.) 



Classification and nomenclature. — Owing to the very slight develop- 

 ment of the accessory minerals mica, hornblende, etc., it has been 

 found impossible to adopt the system of classification and nomencla- 

 ture used with the granites and other rocks. Vogelsang's classifica- 

 tion as modified by Rosenbusch is based upon the structure of the 

 groundmass as revealed by the microscope. It is as follows : 



Groundmass liolocrystalline granular Micro-granite. 



Groundmass liolocrystalline, but formed of quartz and feldspar, 



aggregates rather than district crystals Granophyr. 



Groundmass felsitic Felsopbyr. 



Groundmass glassy Vitrophyr. 



Intermediate forms are designated by a combination of the names as 

 granofelsophyr, felsovitrophyr, etc. The name felsite is often given to 

 rocks of this group in which the porphyritic constituents are wholly 

 lacking, as in specimens No. 38031 and 69571, from Saugus and Nahant, 

 Massachusetts. The names felstone and petrosilex are also common, 

 though gradually going out of use. Elvanite is a Cornish miners' 

 term and too indefinite to be of great value. (Specimen 36188, from 

 Cornwall, England). Eurite, now little used, applies to felsitic forms 

 such as specimens 38245, 38254, and 38259, from Jura, France. The 

 name felsite-pitclistone or retinite has been given to a glassy form with 

 pitch-like luster such as occurs in dikes, cutting the old red sandstone 



