252 PROCEEDINGS OF THE ACADEMY OF [1880. 



October 22, 1871. 



A New Locality for Analcite — Dr. A. E. Foote called atten- 

 tion to the fact that Analcite had been found at Falls of Schuyl- 

 kill, — a new locality for that mineral. 



November 26, 1877. 



On the Measurement of Plave Angles. — Mr. Lewis described a 

 simple and quick way of measuring plane angles in minerals. It 

 was a method which he had found very useful in the measurement 

 of all edge angles, of cleavage and striatiou angles, the angles 

 of markings and dendrites in mica, and of other flat angles to 

 which a goniometer could not conveniently be applied. 



A paper protractor was constructed, the radii of which, distant 

 each from each 1°, were continued from the circumference 

 to the centre. Horizontal lines, about twenty in number, are 

 drawn across these, parallel to the radius 0° and at right angles 

 to the radius 90°. These lines being parallel, the angles formed 

 by the intersection of any radius with each of them are equal. In 

 order to measure the angle of a crystal, it is laid on the protractor, 

 one of its edges is made parallel to a horizontal line, and then the 

 crystal is slid along that line until the other edge, forming with 

 the first the angle to be measured, becomes parallel to one of the 

 intersecting radii. The desired angle is now read oflT on the cir- 

 cumference of the protractor. Angles approaching 90° are read 

 on one of the upper horizontal lines, while those of less amplitude 

 are read correspondingly farther down. A magnifying lens is 

 conveniently used to determine the exact coincidence of the edges 

 of the ciystal with the lines of the protractor. Very large crys- 

 tals as well as crj'stals as small as a millimetre in diameter can be 

 measured in this way. 



It was found that this method of measurement was very conve- 

 nient, and, if the protractor had been carefully made, was exact to 

 within 30' ; while it applied to those cases in which neither the 

 reflective nor the hand goniometer could be used. 



December 17, 1877. 



On an Exfoliating Talc. — Mr. Henry Carvill Lewis described 

 a variety of talc, occurring at the soapstone quarry above Mana- 

 yunk, which is in some respects new. It occurs in fan-like crystals 

 in Dolomite, and is much more similar to Pyrophyllite than to 

 common talc. It moreover differs from common talc by exfoliat- 

 ing when held in the flame of a candle or Bunsen bui'ner, and 

 was, therefore, at first mistaken for Pyrophyllite. In the closed 



