1892.] NATURAL SCIENCES OF PHILADELPHIA. 325 



great masses had fallen in, forming at one point one of the places 

 of historical interest known as Betty Moody's Cave. The length 

 of this dyke from shore to shore is probably five hundred feet, but 

 of this about two hundred feet have been eroded to sea level. In 

 the middle the trap is about ten feet below the gneiss walls; to the 

 south it descends by a series of steps quite abruptly and between 

 vertical walls of the gneiss probably fifty feet to the sea. 



The present erosion continuing, this southeast end of the island 

 will be cut off entirely and form a separate island of the group. 

 At other points the same action may be seen. 



Two phenomena were observed for which no easy explanation 

 offers. The dark, fine-grained gneiss was usually rather regularly 

 inter-stratified in the more abundant coarse granitoid gneiss, but at 

 a number of points it was observed abutting upon the gneiss in the 

 direction of the strike, but without the slightest evidence of a fault. 

 At one point on Appledore Island there was a stratum of the dark 

 variety, thirty feet wide ; suddenly, and almost at a right angle, 

 twenty feet of this were replaced by the coarse granitoid rock, while 

 the remaining ten feet went on as before. A clue to the explanation 

 was seen on Appledore, where a stratum of the fine-grained was 

 separated from a larger mass of the same of darker color by a foot 

 or two of the coarse feldspathic rock, which also bounded it on the 

 further side. This stratum had, in about forty feet, five constric- 

 tions, narrowing it from three feet or more to hardly as many inches 

 in one place. 



The other feature was "a form of erosion which he had never before 

 seen. On the horizontal or slightly inclined surfaces of the nearly 

 vertical, fine-grained gneiss were numerous holes, from the size of a 

 small pea to that of a cherry. The gneiss was hard and undecom- 

 posed. These pits were roughly globular and were generally larger 

 below than at the opening. They appeared to enlarge and coalesce, 

 thus breaking down the rock, bearing a slight resemblance, on a very 

 small scale, to the pot holes of a river bottom. These were high 

 above the sea. They appeared somewhat as if a mineral, like gar- 

 net, had weathered out, but there are no such minerals in the rock, 

 and the holes show no evidence of such ; they are quite rough on the 

 inside and hence have not been bored. Their position is such that 

 only ocean spray and rain water could reach them. They are quite 

 abundant. 



On the Permanent and Temporary Dentitions of certain Three-toed 

 Horses. — Professor Cope described the changes in the characters of 

 the superior molars of Protuhippus placidus Leidy, resulting from 

 age and wear, and the characters of the dentition of colts of Proto- 

 hippus and Hippotherium. He pointed out that in stages of wear 

 up to middle life P. placidus is the Hippotherium gratum of Leidy, 

 and that then the protocone fuses with the paraconule, and the ani- 

 mal becomes a Protohippus. . He had not observed this to take 



