72 Dr Colquhoun on the Argillaceous Ore of Iron. 



In many respects the geological habitudes of this ore are 

 very analogous with those of the other minerals of which the 

 coal basins are composed : they are distinguished however by 

 some peculiarities of their own. Thus the ore generally occurs 

 in regular strata, more considerable in point of number than 

 extent. In ordinary cases, the strata dip from the directly 

 horizontal position, with a declination varying from one yard 

 in four, to one in ten. But there is no particular angle of in- 

 clination to which they are limited. They are sometimes found 

 lying quite horizontal, and, on other occasions, where the for- 

 mation has undergone some violent convulsion, they suddenly 

 rise up in a direction which is nearly vertical. Faults or dykes 

 also occasionally make their appearance in the strata of iron- 

 stone, as in the other strata which compose the independent 

 basin, and these, without their occurrence being always refer- 

 able to any adequate apparent cause, have the effect of abrupt- 

 ly breaking the continuity of the bed of ore, and of either ele- 

 vating or depressing the whole succession of strata, however 

 they may be composed, on one side of the break, for several 

 hundred yards out of their natural course. In this manner 

 these dykes completely disjoin what seems formerly to have 

 been connected, and severely perplex the miner whose opera- 

 tions are often thereby brought to a sudden and unexpected 

 pause.* 



It seldom happens that a single stratum of the ore is disco- 

 vered by itself, and apart from all connexion with other beds 

 of ironstone. On the contrary, the strata which occur toge- 

 ther are often very numerous, varying from about ten to thirty 

 or forty, in the same tract of country, but all of them differing 

 both in the chemical and mechanical constitution of the ores 



• It may be observed that the manner in which the miners use the term 

 di/ke is not strictly accurate, for they apply it indiscriminately to every dis- 

 turbance which takes place in the equable and progressive extension of 

 strata. In its correct application, however, the term is nearly synonymous 

 with vei7i, and denotes a vertical fissure which is filled up with some fo- 

 reign mineral. An abrupt elevation or depression of strata, unaccompanied 

 by the interposition of any extraneous body, is, properly speaking, a slip. 

 The very common occurrence of faults compounded of the slip and d;yke 

 has probably been the cause of these two terms being so frequently con- 

 founded. 



