236 Dyke on Clifton Down. 



originated iu consequence of the highly comrainnted state of the carboni- 

 ferous limestone at this spot ; the cracks and fissures being filled with 

 variously-coloured calc spar, hsematitic iron ore, sulphate of barytes, and 

 crystals of quartz, giving it, on a cursory view, the appearance of a brec- 

 ciated rock. 



Such errors often become established and perpetuated by various authors 

 who copy from each other, and although in themselves of little consequence 

 taken generally, become important when considered locally 5 embarrassing 

 the student, who surrenders his own judgment to the authority of those 

 who first made the mistake, and to the apparent corroboration of those 

 who have in fact merely copied the error from what they have considered 

 good and sufficient authority. 



This wall of rock appears to have originated from the accidental circum- 

 stance of two considerable vertical fissures having occurred parallel to each 

 other, within the short distance of the thickness of the wall, those por- 

 tions of the rock both to the eastward and to the westward having been 

 quarried away, leaving the intermediate rock standing. 



Such fissures or veins are not uncommon, running parallel to each other, 

 constantly in the direction of the present magnetic axis, viz. from north to 

 south, while smaller veins may be traced branching from them in different 

 directions, filled with beautifully- coloured spar, and containing occasionally 

 the bones of herbivorous quadrupeds. 



Two more such fissures may be distinctly traced at a small distance to 

 the westward of that which has been considered as raagnesian, of which 

 formation indeed not the slightest portion can be traced on the observatory 

 down, while on the less elevated down of Clifton, particularly at the section 

 made by the new road, the formation is exceedingly well shewn, the con- 

 glomerate reposing horizontally on tiie inclined strata of the upper limestone 

 shales and grits, thrown down by the great fault. 



Another favourable situation for the observation of these vertical veins is 

 in the quarry called the great white quarry, ou Durdham Down, in one of 

 the veins in which may be detected angular fragments of a white oolitic 

 limestone, cemented by calc spar and galena crystallised in ex-cubes.* 



Yours truly, 



S. STUTCHBURY. 



IssTiTUTioK, Sep. 4, 1835. 



• S])ecimens of which may be seen in the museum of the Institution. — Ed. 



J 



