Crosby.] 248 [Jan. 6, 



structure is more or less noticeable in the natural ledges for some 

 distance on either side of the railroad ; but it is well exposed, so 

 far as I have observed, only in the cuts on the railroad midway be- 

 tween Needham Plains, and Charles River Village, and near the 

 southern margin of the felsite area, the columns showing along the 

 track for about half a mile. The columns, the observed lengths 

 of which range from three to twenty feet, neither of the natural 

 terminations being exposed, are always straight, and have at all 

 points a very uniform inclination to the north, making an angle of 

 about 10° with the vertical. They are from four to twelve inches 

 in diameter, the greater number being from six to eight inches. 

 The great majority are either exactly or approximately hexagonal 

 on the cross section, although the extreme range is from three- 

 sided to eight or nine-sided columns. They seem to exhibit all 

 the essential characteristics of basalt columns ; and, although the 

 masses of felsile in which this structure is well developed alter- 

 nate with, and shade off gradually into, those in which it is less 

 distinct or wholly wanting, the evidence is perfectly decisive 

 against the view that the columns are due to the intersection of 

 ordinary or parallel joints. The cracks plainly circumscribe the 

 columns, and are not, in any observed case, parallel or intersect- 

 ing. Although the development of this structure in sedimentary 

 rocks, by dessication, is conceivable, it is certainly extremely rare, 

 if not unknown, in rocks the aqueous origin of which is unques- 

 tioned. Hence it seems better to regard this columnar jointing as 

 identical in origin as well as in characteristics with that of the ba- 

 saltic rocks ; especially since the felsite exhibits no other features 

 inconsistent with the view that it is a great lava flow or series of 

 flows. 



The columnar structure of this rock is mentioned in my "Con- 

 tributions to the geology of eastern Massachusetts," 1 but, regard- 

 ing the felsite at that time as a sedimentary rock, I failed to appre- 

 ciate its full significance and importance. 



General Meeting, Jan. 6, 1886. 



The President, Mr. S. H. Scudder in the chair. 

 The President announced the death of Prof. C. E. Hamlin of 



iOcc. Papers Boston Soc. Nat. Hist, in, p. 80. 



