GNEISS. 151 



gneiss, and, in Isla, it alternates with the gneiss, or 

 forms beds, in a series or mass, in which itself is the pre- 

 dominant rock. In many parts of Perthshire, the gneiss 

 often alternates with heds of limestone, too considerable 

 to be reckoned subordinate to it ; since, in some of the 

 examples, they extend for twenty miles, with scarcely 

 an interruption. I have elsewhere shown that it alter- 

 nates also with the primary red sandstone. If gneiss 

 thus succeeds to every primary stratum, so, in Suther- 

 land and Shetland, it is immediately followed by the old 

 red sandstone, and, in Morven, by the lias and subse- 

 quent strata containing coal, forming the brief geolo- 

 gical series formerly quoted. I may lastly remark, 

 that the gradation of gneiss into chlorite schist, and ul- 

 timately into the red sandstone, in Sky, is not less in- 

 teresting than those into quartz rock and micaceous 

 schist ; and that its whole history, as now given, may 

 serve to prove under how little knowledge geologists 

 have hitherto been establishing canons for the order 

 and relations of the primary strata. 



From former general enquiries, and from the present 

 history, the theory of gneiss ought now to be apparent. 

 Nature, our guide where direct experiments fail, 

 teaches us, that, from the most recent bed of sand or 

 clay, through the whole series downwards, there is no 

 chasm among the stratified rocks to mark where a me- 

 chanical deposit ends, and one of a mixed character 

 begins ; as, from this to the most perfect crystalline 

 stratum, there is an equal absence of all decisive marks 

 of the cessation of one process and the exclusive reign 

 of another. We also find that approximate beds often 

 exhibit a disposition purely chemical, alternating with 

 one partly or purely mechanical ; and that, even in 

 the same rock, a given bed will present an instance of 

 both characters in association. Hence, the mineral 

 composition of gneiss presents no argument against its 



