CRYSTALLINE STRUCTURES OF ROCKS. 173 



as they have been called, seem to be in every respect 

 analogous, and to admit of the same reasoning. This 

 modification occurs on the large scale in Arran, as 

 well as in Orkney ; the prisms being of large diameters, 

 but divisible by transverse joints into very flat tables, 

 and marked by other peculiarities described in the 

 account of this substance. (Chap, xxxviii.) 



In trying to explain the origin of this structure in 

 the last named rocks, it is first to be remarked, that 

 the appearance is limited to a small portion of ex- 

 tensive beds which elsewhere preserve their natural 

 characters; and that, in both, particularly in the 

 sandstone, there is a simultaneous change of the 

 mineral character of the rock. The sandstone passes 

 into jasper; that being evidently the case only, where, 

 from being intermixed with clay, and thus approaching 

 to shale, it is of a compound nature. The simple 

 strata that are found in it are indurated; and the 

 purer sandstone is also hardened, so as to resemble 

 some of the varieties of quartz rock. These are 

 precisely the changes that take place in similar sand- 

 stones where they are found in contact with trap 

 rocks ; appearances so well known that it is unneces- 

 sary to name any examples except that in Salisbury 

 Craig near Edinburgh, and that described by myself 

 at Stirling castle: (Geol. Trans.) 



It is well known that the masses of trap incumbent 

 on the upper strata, are often so entirely removed as 

 to leave no traces of their existence ; and near Dunbar, 

 there are numerous detached portions of these rocks, 

 which have probably been once connected into a conti- 

 nuous mass. It is not therefore unreasonable to suppose 

 that such a mass may have once covered the portion 

 of this sandstone which has undergone that change 

 to jasper which, in other cases, these are known to 



