PHILOSOPHICAL SOCIETY OF WASHINGTON. 109 



dividual cases with the average of the corresponding observed 

 gradients, as deduced from the isobars, taking account of the 

 distances of the stations from the centres of the cyclones. 



Mr. G. K. Gilbert made a communication 



ON THE AGE OF THE TONTO SANDSTONES. 



(ABSTRACT.) 



The author discussed the age of a group of rocks exposed in 

 the grand canon of the Colorado, and locally designated as the 

 Tonto group. After mentioning its reference to the Silurian by 

 Dr. Newberry, and to the Carboniferous by Prof. Powell on strati- 

 graphical grounds, he detailed some evidence, paleontological and 

 lithological, which led him to refer it to the Primordial division 

 of the Silurian, and pointed out the interest that would attend 

 the recovery of a fauna from some still lower beds discovered by 

 Prof Powell under the Kaibab Plateau. 



(^This communication will appear in full in the Geological portion of the Re- 

 port of Lieut. Wheeler's Explorations west of the one hundredth meridian.) 



Mr. Cleveland Abbe read a paper 



ON the POSITION OF THE PLANES OP CERTAIN NEBULAE. 

 (abstract.) 



The author stated that he had endeavored to pass from the 

 consideration of the apparent distribution of the nebulas in space 

 up to the solution of more definite and tangible problems. 



He had made a study of the nebulae described in Sir John 

 Herschel's " General Catalogue" as very much or exceedingly 

 extended, and had computed the position in space of a line per- 

 pendicular to the planes of these nebulae, supposing the latter to 

 be thin disks seen edgewise. 



He found that these lines appear to lie in or near a plane in- 

 clined about twenty degrees to that of the Milky Way. 



(TViis memoir will be published in full in the American Journal of Science 



and Arts.) 



Mr. E. B. Elliott made some remarks 



ON THE credit OF THE UNITED STATES, AS SHOWN BY THE VALUE OF 

 ITS SECURITIES. 



