MUSEUM OF COMPARATIVE ZOOLOGY. 



209 



The wall of the Great Basin rises highest above its floor on the south, 

 becoming gradually lower ou the cast and west sides. The height of 

 West Peak above the Basin Pond, as determined by the mean of six 

 pairs of simultaneous observations taken with a Crcen's mountain ba- 

 rometer at the level of the pond, and with an aneroid upon the peak, 

 was found to be 2,287 feet, which is the depth of the basin, measured 

 downward fi'om the top of the same peak. Its walls ou the south and 

 east are so steep that they have never been climbed, except at one or 

 two points, as an act of foolhardy daring. The height of Pamola above 

 the little pond, as estimated from a single pair of simultaneous observa- 

 tions, is 1,891 feet, or 39G foot less than that of the highest peak. 



It is within the walls of the Great Basin, and upon their summits, 

 that the geology of Ktaadn can best be studied. The whole mountain, 

 from the lowest point where rock in place has been discovered, is com- 

 posed of granite. Of this, five specimens, numbered 3, 5, 23, 25, and 

 57, have been examined by the lithologist of the Museum, Dr. Wads- 

 worth, whose notes upon them ai-e here given. In a general description 

 of the mountain, it may be said that it is made up of two varieties of 

 granite, the gray and the red. To the first variety specimen 3 be- 

 longs ; to the last, belong 23, 25, and 57; 5 being in a manner in- 

 termediate between 3, on the one hand, and 23, 25, and 57, on the 

 other. 



Dr. Wadsworth* s Notes. 



Kg. 3. 



— A gray granite, composed of feldspar, quartz, and biotite. The 

 feldspar is of two kinda; a grayish-white one, with a piukiah tinge, is the most 

 abundant, wliilc subordinate to it occurs a milk-white striated feldspar. The 

 powder of the rock is magnetic. Under the microscope the thin section is seen 

 to be composed of orthoclase and pkigioclasc, quartz, biotite, and magnetite. 

 The orthoclase is much decomposed and cloudy, showing only feeble pohxriza- 

 tiun. The plugioclase, in general, is much less altered, and shows its triclinic 

 character well in polarized light. Some of the crystals, however, show the 

 characteristic banding only in places, principally at the ends and sides of the 

 crystals, while in the more altered portions the twiuucd structure is rarely seen. 

 This alteration renders it very uncertain that the supposed orthoclase is really 

 all so ; and we feel that this has been a fruitful source of error in the micro- 

 Bcopic examination of rocks. The quartz contains numerous fluid inclusions, 

 moving bubbles being seen in them ; it also contains trichites and minute 

 crystals. These crystals are probably apatite and zircon. Some apatite crys- 

 tals were seen in the feldspar. 



No. 



0. 



A pinkish gray granite of the same composition as No. 3, the dif- 

 ference in color being due to the deeper color of the feldspar. It shows in the 



VOL. VII. — NO. 5. 14 



