1823.] Mr. Farey on the Pontefract Sandstone Rock. 271 



the rock in question belongs to, and is the lowest of, the uncon- 

 formable, magnesian, and gypseous series,* which locally covers 

 and forms the eastern boundary of the Yorkshire coal-field, and 

 that this Pontefract rock has been improperly associated and 

 linked with the six other coal-measure grit rocks, named by Mr. 

 Smith on the margin of his map. I agree also with Mr. C. in 

 thinking, that this Pontefract rock deserves a very careful exa- 

 mination and description. Towards the accomplishment of 

 which object, 1 beg to mention as the results of much labour 

 and thought bestowed on the investigation of particular rocks, 

 two general maxims, which I submit, should always be kept in 

 view by the practical geologist, viz. 



1 . That an individual rock or stratum is never uniform through- 

 out its whole extent; that is, none of them display at every 

 point the same thickness, composition, or state ; nor do their 

 beds or fissures contain everywhere the same organic or other 

 extraneous matters. 



2. That no unique parts occur locally, in any rock or stratum, 

 except, perhaps, in places where an alluvial covering may have 

 infiltered or otherwise communicated its accidental properties ; 

 that is, whatever qualities or states, the beds of any individual 

 rock or stratum may in one place exhibit, or whatever imbedded 

 or venigenous matters it may in one place contain ; each and 

 every of such qualities, states, and matters (with the exception, 

 seldom difficult to discriminate, which has been mentioned above), 

 and the same beds and thicknesses, nearly, may be sought for, and 

 with some confidence expected to occur, in numerous other places, 

 in the whole course and extent of such individual rock or stratum. 



Which maxims suggest forcibly, that much remains yet to be 

 done, which the concurrence of numerous observers can only 

 accomplish, before all the properties of any individual rock or 

 stratum, and their relative frequency of occurrence, being known, 

 the most essential characters can be selected (among which the 

 precise species or varieties of organic remains, will mostly stand 

 foremost, 1 believe), whereby to identify detached and far distant 

 parts, of such individual rock or stratum. 



In the pages referred to in the note below, I have shown, that 

 the Pontefract rock of sandstone, often in so incoherent or loose 

 a state as to be dug for nand, shows itself almost uninterruptedly 

 at the western edge of the yellow or magnesian limestone, across 

 the counties of Nottingham, Derby, York, Durham, and part of 



* On this point, viz. as to the immediate contact always of the Pontefract rock with 

 the lowest magnesian limestone rock, my falling into error in 1807, and (thanks to Mr. 

 William James as to the Bedworth case, and to Mr. James Porter as to the Garforth 

 cage) after seven years setting myself right again, respecting the unronformablentss of 

 the series overlying our coal-measures, had with me no effect ; as will appear in pages 

 I ")7 , fad 468 to 4fij of my first volume of " Derbyshire Report ; " and pages I G8, 409, 

 410, &c. of the third volume; also in the Philosophical Magazine, vol, xlv. pages 167, 

 Mi*!, 174, :j4'2, &c. ; and vol. xlvi. p. ^83, &.c. wherein 1 have correctly represented 

 this important fact. 



