1894. PROCEEDINGS OF THE NATIONAL MUSEUM. 663 



Diorite porpJiyrite. — About six miles northwest from Three Forks, 

 Jefferson Coiiuty. The eriiptives occur here in the form of three 

 api)roxiinately parallel ridges. The outcrops are not continuous, but 

 form a series of rounded knolls covered with scanty soils through 

 Avhich project the angular or rounded fragments into which the rock 

 weathers. 



The most easterly of the three ridges shows outcroi)s in large 

 rounded masses of a coarse gray and pinkish granitic-api)earing diorite 

 in which black hornblende and pinkish or gray feldsi)ars are easily 

 recognized by tiie unaided eye. This, separated by wide rav' ines and 

 benches, is succeeded by a compact dark gray tine-grained micaceous 

 rock in which only small scales of black mica in a very hnely granular 

 base are recognizable, .and this in its turn by a very ty])ical diorite 

 porphyrite, a dark gray very compact rock thickly studded with black 

 hornblendes of all sizes up to 15 or 20 mm. in length, and often in 

 stellate clusters of radiating individuals some 25 mm. in diameter. 

 The held rehrtionshii)s of the last two varieties were somewhat obscure, 

 but although never observed grading into one another, little doubt was 

 felt at the time but that they were portions of the same mass. 



In tliin section the tirst mentioned (No. ToLTO, U.S.N. M.), the graii- 

 ite-lfke rock, is fouml to consist of hirge i)lates of muddy and impure 

 orthoclase and jdagioclase feldsi)ars with interstitial quartz, deep 

 green hornblendes, ami occasional light-green augites, scattering apa- 

 tites, si)henes, an. occasional zircon ( ?), and the usual iron ores. The 

 second variety, the compact tinely granular rock with microscopic'inica 

 shows in the section a finely holocrystalline grouudmass of stout idio- 

 morphic plagioclases and orthoclase in broad plates with abundant 

 sprinklings of green hornblende, paler green augites, brown mica, iron 

 ores, apatites and sphenes. A ])art of the hornblendes are original and 

 a part secondary after the augites. The rock is not distinctly ])orphy- 

 ritic, and the structure as a whole is panidionu)rphic. Occasional large 

 plates of a nonstriated feldspar inclosing small augites and plagio- 

 clases give rise to o])hitic forms. The thir<l, the por])hyri tic variety, 

 shows a similar mineral comi)osition, but somewhat variable structure. 

 Certain slides show a dense inicrograuular feldspathic base carrying 

 occasional rounded blebs of quartz and ])henocrysts of plagioclase and 

 dee]) green hornblende and smaller augites in good idiomorphic forms; 

 others show a structure almost granitic and with interstitial quartz. 

 Hornblende occurs both as i)henocrysts and as a constituent of the 

 groundmass, but in the latter case is always an alteration product of 

 the augite. Mica in this variety of the rock is much less abundant 

 than in the last, and is at times almost wholly lacking. All inter- 

 mediate grades of structure exists from the close-grained porphyritic 

 to the granitic, and the mass as a whole, if as sui>posed all portions 

 belong to the same magma, offers an interesting tield for those who are 

 disposed to make structural differences a basis for rock classification. 



