Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/81

 the position of these rocks, refer them to the syenite family, although had I met with the same specimens connected with a mountain of granite, and lying under mica slate, I should have referred them to the granites.

This is far from being the only case in nature where mere mineral distinctions are insufficient to determine the geological situation of a rock. In the stratified classes of rocks, both primary and secondary, these resemblances are frequent, since it is often impossible to distinguish quartz rock from sandstone, the breccias which it contains from the more recent graywacké, ancient clay slate from recent, or, as I have shown in this very account of Sky, primitive from secondary limestones. The same rocks seem in some cases to have been repeated at different epochas, while in others they show variations which may perhaps be the results of posterior changes operating on the first deposits rather than the consequences of original differences.

Two other varieties of this rock occurred to me which may be mentioned, although possessing no peculiar interest. In the one chlorite formed a constituent part, and in the other a greenish compact steatite was intermixed with the felspar and hornblende, the total compound being not much unlike the porcelain granite of Cornwall.

Before I conclude these remarks on the trap and syenitic rocks of Sky, it will not be superfluous to enumerate the striking external features in which rocks so nearly associated differ.

The mountain trap of the Cuchullin is most strongly distinguished from the stratified in the difference of its disposition, in the absence of the columnar forms and decomposing tendency, and in the barrenness of its surface compared with the deep soil and highly clothed vegetable surface of the latter. In its superior permanence, a permanence