W, S. Bayley — Soda- Granite, </<•., of Pigeon Point. 878 



cent — and that observed In the present case at the same tem- 

 perature — 14*38 per cent. In the latter the drying and heat- 

 ing at continually rising temperatures had continued during a 

 period of 528 hours, the weighings being made usually from 



day to day, whereas in the former case the duration of the ex- 

 periment was much shorter and the intervals between weigh- 

 ings were hut a few hours each. 



2. Crystalline form of Ityrolite, by E. S. Dana. 



Among the specimens of tyrolite recently received from Dr. 

 Hillebrand, there are several which are crystallized with suffi- 

 cient distinctness to allow of a somewhat more complete deter- 

 mination of the form than has hitherto been possible. The 

 crystals are in thin tables or scales flattened parallel to the 

 basal plane of cleavage, and they are united by one edge so as 

 to form radiating- fan shaped groups having a rounded and 

 deeply striated exterior. Occasionally minute isolated crystals 

 can be observed. These are seen under the microscope to be 

 bounded by the pinacoidal edges parallel to which extinction 

 takes place ; further, the bisectrix is found to be sensibly nor- 

 mal to the tabular plane of cleavage, so that they can be re- 

 ferred to the orthorhombic system with little question. Besides 

 the pinacoids, the crystals are bounded by two prisms, 110, and 

 1*20. the former having a normal angle in front of S6°, while 

 the latter is inclined to 010 at an angle of 28°. From the first 

 angle we calculate : 



a: & = 9325; 120 ~ 010 = 28° 12' 



Xo planes were observed which could fix the length of the 

 vertical axis. The crystals are elongated parallel to the macro- 

 axis and the optic axes lie in the brachydiagonal section, the 

 bisectrix being normal to the base ; the character of the double 

 refraction is negative and the axial angle large. 



Art. XXXY. — The Origin of the Soda- Granite and Quartz* 

 Keratophyre of Pigeon Point / by W. S. Bayley. 



(Published with the permission of the Director of the U. S Geological Survey.) 



In tins Journal for January, 1889,* the writer described a red 

 rock from Pigeon Point, Minnesota, with the characteristics 

 sometimes of a soda granite, sometimes of a quartz-keratophyre. 

 The first is a fine grained, drusy rock, speckled with little spots 

 of a dark-green color. It is composed essentially of a hypidi- 

 omorphic granular aggregate of two feldspars, quartz, chlorite 



*Jan., 1889, pp. 54-63. 



