Dr Colquhoun on the Argillaceous Ore of Iron. 75 



half a cubic foot of this bitumen. Mr Mushet has remarked 

 that the bitumen is of rarest occurrence in those nodules in 

 which the calcareous spar is most abundant. 



The Hatchetine, lately discovered by the Reverend Mr 

 Conybeare, appears to be a very pure form of this bituminous 

 substance. It was found " filling small contemporstneoua veins 

 lined with calcareous spar and small rock crystals in the iron- 

 stone of Merthyr Tydvil." The most remarkable feature dis- 

 tinguishing it was its colour, which was yellowish-white. It 

 had the external appearance, and many of the mechanical pro- 

 perties of wax ; being soft, inodorous, inelastic, and melting 

 at a very low heat. * 



Such are the general geological characteristics of the argil- 

 laceous carbonate of iron. On looking at the share which the 

 carbonate of iron occupies in the crust of the earth, it seems 

 susceptible of division into three leading classes. To the first 

 of these may be assigned all those important ores which occur 

 in primitive or transition formations. They are characterized 

 by a crystalline structure, and must all be ranked as varieties 

 of the sparry iron ore. To the second class may be referred 

 the ores which present themselves in the secondary formations. 

 These belong, for the most part, to the coal basins, but strata 

 of them are also found, though very rarely, in the calcareous 

 beds which lie immediately above these formations. They con- 

 stitute the argillaceous ores, which are the peculiar object of 

 this memoir. To the third class remain the very scanty exam- 

 ples of those ores which are found in alluvial districts. It is 

 probable that the ores found in such situations will fall to be 

 classed sometimes along with the sparry iron-ore, sometimes 

 with the argillaceous carbonate. 



Having now pointed out the general characters of ironstone 

 viewed as a mineral, and subdivided it into its several minera- 

 logical varieties, and having also given a brief notice of its geo- 

 logical history, we may proceed to consider it, in reference to 

 the chemical nature and affinities of the various ingredients 

 which enter into its composition. This indeed is the most im- 

 portant point of view in which we can regard a mineral which 



* Annals of Philosophy ^ New Series, i. 136' 



