622 CENTRAL MINERAL REGION OF TEXAS. 



and more highly siliceous than in places where the hard ore bands pass be- 

 neath them. This principle can not be carried to an extreme in a practical 

 way because there is only a gradual diminution in the ferruginous contents 

 of the sandstones as one passes upward through the Cambrian beds; and the 

 Lower Cambrian strata as a whole are more heavily charged with iron than 

 are the Middle and Upper Cambrian series. But to one who has acquired a 

 certain familiarity with these beds there are special criteria by which the com- 

 mercial values may be roughly determined. 



Prior to the deposition of the Cambrian strata the Central Mineral Region 

 had been eroded extensively, so that the conglomerates and sandstones which 

 formed along the early Cambrian seashore in this area were deposited upon 

 an uneven surface, exposing patches of each of the three preceding systems. 

 In some places, no doubt, the magnetites of the Iron Mountain series were 

 thus left uncovered, so that they became a prey to the waves, were commi- 

 nuted by them, and the sandy product was afterwards redeposited alone or 

 with the material from the degradation of the associated rocks. In this way 

 somewhat indeterminate accumulations of magnetic and hematitic sandstones 

 may be explained. These lie at different points along the tortuous border of 

 the Cambrian outcrops, and no definite shape or extent to any exposure can 

 be asserted without detailed examination. Some of the general areas in 

 which this class of ore is prevalent are roughly plotted upon the map accom- 

 panying this Report, but it would be impossible to show them in detail with- 

 out devoting a whole season to the special study of their distribution. A 

 few general specifications will be all that is needed to indicate their nature 

 and the mode of determining their value in individual cases. 



1. The sandy ores do not occur in narrow bands or belts, but in disconnected 

 patches, representing the remnants of an old shore line which has been oblit- 

 erated in many places by denudation, and which is buried at other points be- 

 neath a varying thickness of the later sediments. Besides this, the uneven 

 character of the supply rendered the assorting process so irregular that the 

 existing relics can hardly be said to exhibit any readily discernible relation to 

 the positions of the hard ore bands from which they have been derived. 



2. These ores are liable to be more abundant where the hard ore bands have been 

 worn away, provided that the erosion has not taken place since the deposition 

 of the former. 



3. The sandy ores must be worked in local patches, according to individual 

 merit, for the amount and quality are dependent upon so many purely local 

 contingencies that only the most general description can apply to all. They 

 vary so greatly in the percentages of quartz, magnetite, hematite, and even 

 limonite (by alteration), that nothing but the most careful inspection and 

 chemical tests can determine the relative values of different parts of the same 



