Rock-Basins with the Spheroidal Structure of Granite, &$c. 337 



rock to disintegrate more easily in certain directions, with re- 

 spect to the planes of its surfaces, than in others. 



The form and appearance of the rock-basins on the hori- 

 zontal summits and ledges of Roach rocks, and the deep chan- 

 nels on their highly-inclined surfaces, may perhaps be men- 

 tioned as affording some degree of support to the foregoing 

 conjecture. Here the basins are very deep, and their outline 

 is comparatively irregular ; while the channels which the 

 water has worn on the declivities, do not, so far as has yet 

 been described, occur upon granite, under the same circum- 

 stances. Now, supposing the form of the basins, as they exist 

 in granite itself, to be connected with the concentric-sphe- 

 roidal structure of the rock, the facts just mentioned are what 

 we might have expected to find, in a mass of schorl-rock like 

 that of Roach. For although a process of disintegration has 

 undoubtedly taken place with it, and is still proceeding, it 

 tends to produce rather angular and pyramidal, than cubic 

 and spheroidal blocks. From observation, therefore, we ap- 

 pear to have no evidence of the spheroidal structure of this 

 particular mass of schorl-rock ; and this negative result may 

 be confirmed from theory; since the felspar, on the predo- 

 minance of which, in granite, all the phenomena attendant 

 upon and immediately succeeding its presumed original state 

 of igneous fluidity, must have been greatly dependent, is al- 

 most entirely wanting in this rock, which is unusually uni- 

 form in constitution, consisting merely of quartz and crystals 

 of schorl, intersected by a few veins of the former mineral *. 



Another 



* It is referable, with little exception, to the first variety of schorl-rock 

 described by Professor Sedgwick, in his memoir on the formations asso- 

 ciated with the primitive ridge of Devonshire and Cornwall ; viz. " gra- 

 nular quartz-rock, with deeply striated prismatic crystals of schorl, of a 

 coal-black colour, and without regular terminations, uniformly disseminated 

 through the mass." Trans, of Camb. Phil. Soc. vol. i. p. 106. The summit 

 of the eastern rock, only, at Roach, consists of the fourth, or porphyritic 

 variety, described by Prof. Sedgwick, from which, however, the imbedded 

 crystals of felspar have disappeared, leaving the rock in a state resembling 

 a scoria. 



If the concretionary spheroidal structure, in granite, shall be found, here- 

 after, to have resulted from the continued action of heat upon the rock 

 subsequently to its consolidation, as Dr. Macculloch has shown to have pro- 

 bably been the case with the corresponding structure in the trap-rocks, 

 and almost demonstrated to have been so with the columnar iron-stone of 

 Arran, the prismatic sandstone of Rum, and the columnar sandstone of Duu- 

 bar, then the inference, in the text, grounded on the presence of felspar in 

 granite, and its absence in the schorl-rock of Roach, will become nugatory. 

 For although the predominance of felspar, (or that of its constituents,) 

 would certainly have exercised an important influence, in the phsenomena 

 of crystallization, which must have taken place at the consolidation of the 



N.S. Vol.8. No. 47. Nov. 1830. 2X rock5 



