Prismatic Structure in Sandstone, Sfc. 259 



appear to be anywhere jointed, but they break at right angles 

 to their axes. Neither do they present any marks of that 

 peculiar channelling on the surface which occurs in the shale 

 of Arran. 



Independently of this decided configuration, there are some 

 indications of an internal structure in this sandstone bed, 

 connected with the prismatic form, which it is necessary to 

 notice. 



Where the first indication of the future division into columns 

 takes place (see Jig. 1.), the primary or bounding lines by 

 which they are indicated are often filled with a series of con- 

 centric lines, distinguished by some differences both of colour 

 and texture, and commonly by white lines on a red ground. 

 This circumstance presents an interesting analogy to the case 



Fig. 2. Subdivided Prismatic Structure of the Coiumnt at Dunbar. 



of the basalts, in which, from their eventual decomposition in 

 successive crusts, it appears probable that they possess an 



