1885.] 247 [Crosby. 



on the shrinkage hypothesis is obvious for several reasons : In 

 the first place, the cliffs are clay from top to bottom, and yet the 

 joints are entirely unlike the shrinkage cracks observed in dessi- 

 cated cla} T s ; but their intersections without interference, parallel- 

 ism, and regular interspaces are just the characteristics belonging 

 to earthquake fractures in such homogeneous, inelastic mate- 

 rial. Again, there is no evidence that the clay has suffered 

 shrinkage from dessication or any other cause. The joints are 

 not open cracks, but mere planes of division, the opposing sur- 

 faces being in close contact. Besides, this formation probably 

 always has been essentially superficial, i. e., it has not been sub- 

 jected to a high temperature or pressure, the diminution of which 

 would be essential to contraction in the absence of dessication. 

 But the clay certainly has not lost volume by drying, for it is still 

 a soft plastic clay ; and the joints are most marked at the bottom 

 of the cliff, where the clay is saturated with water. 



I wish next to call attention to a good example of columnar or 

 prismatic jointing in the felsite of this vicinity. Although this 

 throws no light, so far as I can discover, upon the origin of joint- 

 structure, it appears to be of some interest on account of the na- 

 ture of the rock in which it occurs, and, in my opinion, tells quite 

 strongly in favor of the view that our felsitic rocks are eruptive. 



In the town of Needham, Mass., there is a large, well defined 

 and isolated area of felsite. The felsite begins in the southern 

 part of Newton, crosses the Charles River just south of Newton 

 Upper Falls, and stretches thence nearly across Needham, the ex- 

 treme length being about four miles, and the breadth ranging from 

 one to two miles. The village of Needham Plains is situated near 

 the middle of this area, and south and west from this point, es- 

 pecially, the felsite is well exposed in prominent ledges. It is 

 throughout a greenish gray quartz-porphyry, which is often quite 

 green on the weathered surface, showing a tendency to change to 

 pinite. One of the most remarkable features of the rock is its 

 homogeneity, for certainly no other considerable area of felsite in 

 eastern Massachusetts presents so little variety. 



The Woonsocket branch of the New York and New England 

 Railroad crosses the felsite area in the direction of its length, and 

 it is along the line of this road, southwest of Needham Plains, 

 that the prismatic jointing has been observed in the felsite. This 



