On the Tertiary Iron Ore Measures. 153 



chalk and its whiteness facilitates its manufacture into whiting 

 and bleaching powder ; when burnt it becomes an excellent lime 

 for agricultural purposes. 



In places on the white limestone forming the basal bed of the 

 Tertiary, is a thin ferruginous conglomerate made up principally 

 of abraded flints, clay, and gravel ; but occasionally we find a 

 stiff brownish clay (which much resembles the pavement hereafter 

 mentioned, page 156), having imbedded pieces of burnt-looking 

 flint and limestone, also occasional pisolites of iron ore ; some- 

 times, however, the dolorite rests direct on the chalk. 



The other rocks of the Tertiaries are the dolerites, basalts, 

 and Tuflfs ; under the latter being included the laterites, iron 

 ore measures, bauxite (silicate of alumina), and such like. The 

 dolerites may be divided into upper and lower, or those above 

 and below the main iron ore measures. 



The dolerites and basalts occur in flows, protrusions, and dykes, 

 while associated with them are partings of laterite, the ferru- 

 ginous beds of lithomarge, aluminous and pisolitic iron ores, and 

 seams of lignite. The dolerites are more or less crystalline, 

 sometimes being porphyries, having in them well developed 

 crystals of labridorite and augite, occasionally they exhibit 

 columnar and amygdaloidal structures and have peculiar concre- 

 tionary weathering ; both they and the basalts when in bedded 

 masses occupy large areas, while the tuffs are of more or less 

 limited extent. 



The flows of dolerite vary from a few feet to thirty or forty in 

 thickness ; usually between them are ferruginous accumulations, 

 to which I drew the attention of Mr. Mallet, of the Indian Survey, 

 during a visit to the mine last summer, and he expressed an 

 opinion that the compact varieties were somewhat similar to the 

 laterites that in India occupy large areas, are often 200 feet deep, 

 and contain nodular iron beds twenty to forty feet thick. Here, 

 however, the beds or partings are only from a few inches to 

 about seventy feet, while in general they do not exceed two feet 

 thick. In places these thin laterites graduate into good alumin- 

 ous iron ore, but in no case -have I found the latter assuming the 

 pisolitic structute ; although in the iron ore measures similar 

 aluminous ores will occasionally have pisolites scattered through 

 them when immediately under the pisolitic ore. 



SciBN. Proc, R.D.S. Voi,. III., Pt. IV. 2 



