TIDESWELL DALE QUARRIES. 3 



to go into the question of the production of the prismatic structure, 

 but to give several typical examples. That of the Giant's Cause- 

 way is sufficiently well known. Here, as is frequently the case, 

 the columns are divided at regular intervals by transverse joints, 

 the segments exhibiting a cup and ball structure, doubtless the 

 result of further contraction. In Wales the columns are commonly 

 used for gate-posts. A magnificent example occurs in the Horn- 

 graben Valley, in the Eifel district of Germany. In a quarry cut 

 into a lava stream which flowed from one of the craters of the 

 neighbouring Mosenberg, are to be seen numerous columns, some 

 of which are over three feet in width, and considerably more than 

 one hundred feet in length (Fig. i). 



Fig. i. 



It is worthy of note that this prismatic columnar structure can 

 be produced artificially, and not only as a result of contraction in 

 cooling, but as a result of shrinkage, due to loss of moisture. 

 Fig. 2 is a sketch kindly furnished by 

 my friend, Mr. Ward (for the use of 

 whose notes I am much indebted), of a 

 piece of starch, in which the columnar 

 structure has been developed by drying. 

 He informs me that it was not pro 

 duceii at ordinary atmospheric tem- 

 peratures, but that if a mass of 

 starch, which had been allowed to Fi s- 2 - 



slowly dry, and in which the structure had not been developed. 



