476 ORIGIN OP ERUPTIVE AND PRIMARY ROCKS. 



mica-schists and clay-slates in the districts above mentioned never 

 occur overlying the gneiss strata unconformably. On the contrary r 

 they are so intimately connected that a gradual transition is gener- 

 ally observed to take place between them ; the gneiss gradually 

 changes into mica-schist, the latter gradually becomes less crys- 

 talline, and finally argillaceous and chloritic rocks result. 



A further modification of the above type of the structure of 

 the slate rocks occurs when the granite is so extensively protruded 

 as to overlie the gneiss strata, or when the latter have not been 

 forced up to the surface. In this case the micaceous or argilla- 

 ceous slate is found immediately reposing upon or at least in con- 

 tact with the granite. In this manner the mica-schist with inter- 

 stratified limestones, north of Drontheim in Norway, overlie the 

 granite of Vestfjord, and in this way also the killas or clay-slates 

 of Cornwall lean upon the granite of Dartmoor. In the latter 

 cases no lithological transitions are observable between the 

 slates and the granite, while in former cases, where gneiss is in- 

 terposed between them, the transition from the latter rock to 

 granite is distinctly observable. This phenomenon, it will be 

 observed, however, is not inconsistent with the explanation here 

 given of the origin of these rocks. 



I have thus attempted to explain some of the most remarkable 

 phenomena connected with the primary rocks. It will be obser- 

 ved that in so doing, I have tried to elaborate and combine toge- 

 ther many of the ideas expressed by different geological authori- 

 ties. I am far however from maintaining that the theory here 

 given is adequate to account for all the facts observable in connec- 

 tion with these rocks. Nor is it at present necessary that this 

 explanation should be perfect. There must be in geology as in 

 other sciences, obscure problems always awaiting solution. The 

 best apology which I can offer for presuming to attempt an 

 explanation of the enigmatical phenomena connected with the 

 primary rocks, is in the following words of McCulIoch :* " The 

 human mind is so constituted that it cannot rest content with 

 facts. If it posesses innate propensities, the investigation 

 of causes is assuredly one of them. The very geologist who 

 disclaims all theory has his own ; the lowest of the vulgar 

 desire reasons. The laws which govern the phenomena of na- 

 ture force themselves irresistibly on our attention. They are 

 strictly involved with the analogies which regulate all our reason- 



* System of Geology, vol. i, p. 485. 



