THE QUAETZ POEPHYEIES (55 



(2) THE QUAETZ POEPHYEIES 



Composition. — The mineral and chemical composition of the 

 quartz porphyries is essentially the same as that of the gran- 

 ites, from which they differ mainly in structure. Their essen- 

 tial constituents are quartz and feldspar, with accessory black 

 mica or hornblende in very small quantities; other acces- 

 sories present, as a rule only in microscopic quantities, are 

 magnetite, pyrite, hematite, and epidote. 



Structure. — The prevailing structure is porphyritic. (Pig. 1, 

 PL 2.) To the unaided eye they present a very dense and com- 

 pact ground-mass of reddish, brown, black, gray, or yellowish 

 •color, through which are scattered clear glassy crystals of quartz 

 alone, or of quartz and feldspar together. The quartz differs 

 from that of the granites in having been the first mineral to 

 separate out on cooling it has taken on a more perfect crystalline 

 form ; the crystal outlines of the feldspar are also well defined. 

 Under the microscope the ground-mass in the typical porphyry 

 is found to consist of a dense felsitiCj almost irresolvable sub- 

 stance, which chemical analysis shows to be also a mixture of 

 quartzose and feldspathic material. The porphyritic quartzes 

 show frequently results of marked corrosion from the molten 

 magma, the mineral having again been partially dissolved after 

 its first crystallization. (Pig. 3, PL 5.) This difference in 

 structure in rocks of the same chemical composition is believed 

 to be due wholly to the different circumstances under which 

 solidification has taken place. The structure of the ground- 

 mass is not always f elsitic, but may vary from a glass through 

 spherulitic, micropegmatitic, and porphyritic to perfectly micro- 

 crystalline forms as in the microgranites. This difference in 

 structure may be best understood by reference to Plate 5, which 

 shows the microscopic structure of (1) granite from Sullivan, 

 Hancock County, Maine, (2) micropegmatite from Mount Desert, 

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

 Marked fluidal structure is common. (See PL 2, Pig. 2.) 



Colors. — The colors of the ground-mass, as above noted, vary 

 through 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 ground-mass, forming a desirable stone for ornamental pur- 

 poses. 

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