466 Reviews — Nicholsoris Essay 



beds often imply too much. Some general term for rocks of a por- 

 phyritic structure, without reference to their mineral composition, and 

 for those ash-like beds which graduate into a breccia on the one 

 hand, or gritty sandstones, etc., on the other, without involving any 

 theory as to their origin, would be safer and very useful as enabling 

 a field geologist to state what he actually saw and no more. 



There are two points of view from which we may approach the 

 study of the Igneous and Metamorphic rocks, — 1st. Their strati- 

 graphical relations, i.e. their behaviour as rock-masses, their lie with 

 relation to the strike of the beds among which they occur, their 

 variation in general structure, and the alteration of the neighbouring 

 rocks ; and 2ndly. Their mineral and chemical composition. 



Our author has chiefly confined himself to the former and remarks 

 (p. 46) that all the various granite-masses of Cumberland and West- 

 moreland are strictly confined to the Green Slates and Porphyries. 

 Then, pointing out that this series consists almost entirely of fels- 

 pathic ashes and breccias, with interbedded porphyries, suggests that 

 the fact that the granite occurs always among rocks which themselves 

 show evidence of volcanic activity during the period of their deposi- 

 tion, may throw some light upon the origin of granite. From what 

 follows we see that he implies that the igneous action which caused 

 the volcanic outbursts, to which he refers the ash-beds and porphyries, 

 caused later on in the same area the intrusion of granitic masses. 



Mr. Marshall in an able paper read before the British Association 

 (1858, p. 84) has propounded another view. He holds that there 

 was a given (not a necessarily uniform) depth at which the internal 

 heat of the earth was sufficient to alter or even fuse rocks of a certain 

 mineral character ; that in the unequal depressions to which the 

 rocks of the Lake Country were subjected, owing to the crumpling 

 up of the beds, various parts of the series were carried below that 

 horizon and altered more or less accordingly ; that the granite re- 

 presents the extreme of this metamorphism. 



Dr. Nicholson points out the passage from the granite into the 

 felspathic beds in which it occurs, and speaks of the alteration of that 

 part of the granite which is in contact with the sedimentary rocks. 

 By this, we suppose, he means that the more sudden rate of cooling 

 of the outside of the mass, owing to the lower temperature of the 

 sedimentary rocks with which it was brought in contact, has caused 

 the granite to crystallise there in a rather different manner. 

 Obviously this fact of the passage from the granite to the sedimentary 

 rock is as easily exjolained on Mr. Marshall's view. 



One point seems often to come out from a careful examination of a 

 granitic mass. The granite seems to replace a certain portion of the 

 sedimentary strata, and not to displace them, leaving them pushed 

 out on all sides. If we suppose the intruded rock to eat its way into 

 the sedimentary strata, assimilating portions of it, we allow a good 

 deal of what is asked by those who hold the metamorphic origin of 

 granitic rocks, ^,e., the possibility of changing a sedimentary into a 

 granitoid rock. The advocates of that theory may take their stand 

 upon the assimilated portion, and ask is it the heat of the intruded 



