1880,] NATURAL SCIENCES OF rillLADELPHIA. 279 



dendrites grew was pure white and free from such specks. It 

 seemed that the material of the dendrites is abstracted from the 

 rock and by some segregating force built up into tree-like forms. 

 An examination of their structures showed that the dendrites were 

 quite amorphous and that very frequently the upper extremities 

 of their branches were thicker than the stem portion, as though 

 some concretionary or capillary force acted most powerfully at 

 the growing points. No cr^'stalline structure was apparent, the 

 dendrites being bounded throughout by curved lines. It looked 

 as though they might have grown by a succession of concentric 

 metallic shells. 



It was remarked that these dendrites were quite different from 

 those in muscovite and other crystals, which, frequently derived 

 from the substance of the crystal, have been so influenced by its 

 structure as to become often pseudomorphic. It was noted that 

 there are several distinct kinds of dendrites. They may be internal, 

 as in moss agate ; or external, as in the case now described. They 

 may also be either crystalline or amorphous. The crystalline 

 dendrites are subdivided into those which have been free to 

 crystallize of their own accord, and into those which have been 

 influenced b}' the crystalline structure of the mineral in which 

 they exist. Examples of each were cited. 



On a Jurassic Sand. — Mr. Lewis directed attention to a fine 

 sand of considerable extent and depth, which he had found under- 

 lying the lower Cretaceous plastic clay. If this clay, as is supposed, 

 is the base of the Cretaceous formation, the sand below it may be 

 of Jurassic age. There is a fine exposure of this sand near Elkton, 

 Md. From its coherence it may be regarded as a fine-grained 

 sandstone. It is either white or pale yellow in color, and about 

 16 feet are hei'e exposed. Underneath the plastic cla^' south of 

 Trenton, N. J., the same sand is at least 30 feet deep. It is sug- 

 gested that, in the absence of fossils to fix its age, it may possibly 

 correspond stratigraphically with the " Hastings sand." The 

 overlying clay contains fossils at Baltimore, which Prof. Uhler 

 identifies as Wealden. 



Upon the summit of the same hill, near Elkton, where the above- 

 described sand is exposed, " Bryn Mawr gravel" occurs in abun- 

 dance. It contains " Mt. Holly conglomerate," and has the same 

 features as in Delaware and Penn^lvania. Whether or not it has 

 any connection with the plastic claj^ is not known. This same 

 l)lastic clay, of probably Wealden age, occurs at Turkey- Hill, in 

 Bucks Countv, I'enna. 



5 



May 2G, 1879. 



Potsdam Sandstone near King of Prussia. — Mr. Theodore D. 

 Rand called attention to primal (Potsdam) sandstone rocks in the 

 bed of a valley on the farm of Samuel Tyson, South Chester A'al- 

 ley Hill, near King of Prussia, Montgomery County, Pa. 



