DESCRIPTION OP THE FORMATIONS 375 



"The flagstones retain the same features so well marked in Caithness. 

 Sometimes, as at Skaill in Pomona, they are exceedingly hard, fissile, bitumi- 

 nous, and crowded with fossil fish. In other places, as near Kirkwall, they 

 form thicker beds, and can be quarried for building materials, like those which 

 are similarly used at Thurso. Bands of dull red and even yellowish sandstone 

 occur interstratified with the flagstones, as in Scapa Bay, Meal near Kirkwall, 

 Eday, and other places. Where the flagstones rest on the old crystalline rocks 

 they become for a short space conglomeratic at the base'' (page 409). 



At certain localities occur zones of red sandstone which may be paralleled 

 with the John o'Groat's and Gill's Bay groups of the Scottish mainland. As 

 we advance northwards among the islands the same petrographical characters 

 continue. One "seems to be forever meeting with repetitions of the same rocks. 

 No doubt when these islands come to be mapped in detail, the real thickness 

 of flagstones will be found to be more considerable than might at first have 

 been surmised." The Orkney rocks appear to belong to the upper groups of 

 the Caithness section, since "no equivalents are met with of the massive red 

 sandstones, shales, and conglomerate groups at the base of the Caithness 

 series." "The coarse conglomerate of well-rounded sandstone blocks at Heg- 

 labir on the west side of Sanday, which has been long known (see Barry's 

 "Orkney," page 56, and Neill's "Tour"), seems to occur at a greater distance 

 from the local base, for it is said to overlie sandstones and flagstones" (page 

 410). 



"Many of the flagstones of Orkney are charged with organic remains. Espe- 

 cially is this the case with some of the dark, hard, fissile, bituminous bands. 

 On the surfaces of these strata remains of the characteristic ichthyolites are 

 crowded thickly together, and usually in such a tolerable state of preservation 

 as to show that the fishes must have died where their remains are found, or 

 at least that they could not have been subjected to any prolonged exposure 

 and transport before they were buried under the accumulating sediment. As 

 a rule, the fossils have been converted into a brittle jet-like substance, which 

 is so liable to crack and scale off, that unless great precautions are taken, an 

 organism, which at first showed external sculpture in great beauty, becomes 

 eventually a mere black bituminous patch, retaining only the outline of the 

 original specimen. It is not difficult, in most cases, to distinguish an Orkney 

 from a Caithness ichthyolite. 



"The fossil plants of Orkney include most of the forms found in the Thurso 

 and upper Caithness groups" (page 411). 



Interpretation of the Orcadian deposits. — The structural features of 

 the strata which are significant of origin have been given bv Geikie in 

 more detail than is found in other descriptions. They permit, therefore, 

 of a more specific interpretation. 



Significance of conglomerates. — It has been shown that subaerial con- 

 glomerates may be laid down at much greater distances from the 

 sources of supply than subaqueous conglomerates, since streams may 

 transport resistant gravel until they lose velocity or become loaded with 

 finer material. Subaerial conglomerates may also accumulate with much 

 greater thicknesses than subaqueous conglomerates. The only conditions 



