SECOND AEY STKUCTUKES IN THE CLAYS. 709 



pressed to oiie-fit'th of its former length. Going- northwest, we soon come, 

 at Sunset Hill, on the great drunilins that formed the shore of the ancient 

 lake, and it is clear that the disturbance could not have been caused by 

 ice coming down the valley of Mill River, which lies behind these. 



SECONDARY STRUCTURES IN THE CLAYS. 



Joints. — Where the clay stands in vertical walls in the river l)anks it 

 is in time rudely fissured parallel and at right angles to the exposed sur- 

 face, and as the horizontal seams of sand weaken the cohesion of the mass 

 in the third plane the river in the spring flood often moves oft' bodily great 

 cubical masses of the clay and heaps them up lower down. Several years 

 ago, on visiting the bank of the river below Hadley, I found a Ijroad, 

 horizontal surface thus exposed at about low-water level, which was jointed 

 with extreme regularitv and beauty. The principal lines ran ^jarallel to the 

 edge of the bank, perfectly straight and jiarallel to each other and an inch 

 apart. 



The second set, also parallel to each other, were an inch and a half 

 apart, and made an angle of 60° with the first set. The lines of the second 

 set were not always continuous, sometimes failing between two contiguous 

 lines of the first set, but continued beyond in the same direction. These 

 lines represented fissures which extended tlu-ough one layer of the clay 

 one-third of an inch thick, dividing the clay into blocks of mathematical 

 regularity. Toward the edge the blocks had been moved bv the current a 

 short distance from their original position, manifestly very soon after the 

 superincumbent block of clay had been lifted off, for they were, when I 

 examined them, so soft that they could not be touched Avithout destroying 

 their form, and yet as they lay thev retained their perfect regularity. One 

 could not help thinking that in olden time it would have been called a 

 fairies' pavement, as still in Scotland the claystones are called fairy stones. 



Below, where the large massses of clay had lodged, I found the small 

 blocks piled in considerable abundance, but all softened and fused together, 

 and in subsequent years I have always found them in abundance under the 

 same circumstances. Later I found the same jointing in the large clay pit 

 at the Bay State in Northampton, where surfaces 3 and 4 feet square were 

 regularly jointed, exactly as on the river bank. When a vertical surface 

 had been left for some time and the workmen then attacked tliat portion of 



