414 PROF. T. G. BONNET ON A SERIES OE ROCKS 



highest summit, Cross Fell (2900 feet), being* capped by Coal- 

 measures. The valley below is occupied by Permian and Triassic 

 deposits, faulted down at the foot of the scarp. The average thick- 

 ness of the Permian alone is estimated at between 3000 and 4000 

 feet ; yet not a fragment of it remains east of the fault. If an ex- 

 tensive plateau has been swept clean of 3000 feet of sandstone since 

 the Permian epoch, it is not surprising that another plateau should 

 have been cleared of 300 feet of quartzite since the Ordovician 

 period. 



If it be objected that the stupendous inversions and overthrows 

 which I have described are improbable, I have only to reply that 

 such effects are not uncommon in disturbed districts, and are fami- 

 liar to geologists. I will only call attention to a remarkable 

 example, copied from A. von Heim by Dr. A. Geikie, in his new 

 ' Text-Book of Geology,' p. 518. On the left of the section " schis- 

 tose rocks, perhaps metamorphic Palaeozoic formations," with white 

 Jura conformably underlying, rest upon the upturned edges of 

 highly contorted Eocene strata. 



In the paper which I thus bring to a conclusion, I claim to have 

 invented no new method of investigation. I have worked upon 

 the recognized principles of our science, and they have been found 

 sufficient for my purpose. I shall be rewarded for my labour, if I 

 have succeeded in emphasizing the truth, that the only authority to 

 be recognized by geologists is the authority of Nature herself. 



APPENDIX. 



Notes on a series of Roclcs from the North-ivest Highlands, collected 

 by O. Callaway, Esq., D.Sc, F.G.S. By Prof. T. G. Bonnet, 

 M.A., F.B.S., Sec. G.S. 



In describing this interesting series of rocks I shall endeavour, as 

 far as possible, to gather them into groups in order to save the re- 

 petition of details. A lengthy description will in most cases be un- 

 necessary, because the majority of the specimens belong to types 

 which have already been described with considerable fulness by Mr. 

 T. Davies and myself. 



The first group is a series of coarse, rather granitoid gneisses, 

 generally admitted to be representatives of the Hebridean series. 

 These rocks are rather coarsely crystalline in texture, generally not 

 conspicuously foliated, containing a variable amount of hornblende 

 or of black mica, but occasionally having a white mica as the third 

 constituent. 



Under the microscope, the minerals are well crystallized inter- 

 nally, but do not, as a rule, exhibit externally a definite crystalline 

 form. Those ordinarily occurring are quartz, felspar, mica, and 

 hornblende, with commonly some sphene and epidote, and, occasion- 



