146 TORSA. GEOLOGY. 



occurring on the eastern side of the grey ridge in Luing 

 and in Seil, is, from its limited quantity, held insufficient 

 to prove there, a legitimate alternation between these two 

 classes of rock, that circumstance is fully established by 

 the extent of the former in Torsa ; its horizontal breadth 

 in the order of superposition to the grey ridge, being 

 nearly a mile. 



The stratum of graywacke above mentioned, lies on 

 the east side near the shore, and is of so compact a 

 nature that it is at first sight difficult to discriminate it 

 from trap. In some cases, the specimens, even on minute 

 examination, can scarcely be distinguished from some of 

 the varieties of that family. This is an important fact, 

 when it is recollected how often substances thus resem- 

 bling basalt, are found stratified and interposed among 

 the secondary strata, having apparently originated from 

 the action of heat on the schistose clays; facts already 

 stated at some length in other places, and existing in 

 those, as they do here, in the vicinity of large masses of 

 trap. In this instance the distinction is made by means 

 of the quartz veins, which pass with an uninterrupted 

 continuity from the clay slate through these interposed 

 beds of graywacke ; a circumstance that never takes place 

 in trap rocks. 



Together with these stratified rocks, Torsa presents many 

 rocks of the trap family resembling those of Luing and 

 Seil; but which, being in some respects even more re- 

 markable, were reserved for a more minute detail in 

 this place. 



They are here also found passing through the strata 

 at various angles ; while in some places they put on 

 the appearance of beds, being interposed among these 

 strata in a parallel and conformable manner. In many 

 cases, it is scarcely possible to distinguish them from 

 real strata, either by defect of parallelism or by occasional 

 deviations; yet, even then, they resemble the intersecting 

 veins so perfectly in their peculiar aspect on wcathorinir, 



