2 T1DESWELL DALE QUARRIES. 



constitution differs slightly in different districts. The following 

 are the minerals which enter the composition of that found in 

 Tideswell Dale : — Olivine Augite, and Plagioclase Felspar. 

 Magnetite is also present. In many places, as at Matlock, the 

 rock contains hollow vesicles, produced when the rock was in a 

 molten condition. Water, doubtless disseminated throughout the 

 molten mass prior to its eruption, passed on the withdrawal of 

 pressure consequent upon ejection, into the condition of steam, 

 expanding, and thus producing the cavities. They are common 

 in modern lavas. 



There is very goo 1 reason for thinking that the volcano which 

 gave rise to the rock described was submarine, and in some 

 districts the vesicles are filled with calcite and other minerals, 

 doubtless subsequently deposited from an aqueous solution. In 

 some places the white patches of calcite give to a freshly fractured 

 surface of the rock a peculiar appearance, which has been con- 

 sidered so like the marks on the body of a toad that the rock is 

 known as Toadstone. The name has also come to be applied to 

 the Derbyshire basalt generally. The toadstone (dolerite) in this 

 quarry is particularly interesting, because it well illustrates — on 

 a small scale, it is true — several peculiar phenomena observed to 

 accompany the cooling of volcanic lavas. It is a well-known fact 

 that in the case of almost all known bodies, decrease of tempera- 

 ture is accompanied by contraction. It will be seen that in a 

 stream of molten lava the cooling will not proceed uniformly in 

 all parts of the mass. The upper surface will cool more rapidly 

 than the lower surface, and the surface generally will cool before 

 the interior. There are thus set up in the mass stresses which 

 ultimately overcome the cohesion between the particles, and the 

 stream becomes broken up by a number of divisional planes 

 termed "joints." Under certain circumstances, if the mass be 

 homogeneous, it will, in cooling, split up into a number of pris- 

 matic columns, sometimes of remarkable regularity, and having 

 their axes perpendicular to the main cooling surfaces. The 

 number of sides possessed by the columns are various, but they 

 are usually hexagonal. It is not my purpose in this short paper 





