278 M. 1. Goldman — Catahoula Sandstone of Texas. 



of the source rock and related to the conditions under which 

 the sandstone was formed. For its interpretation, however, I 

 know of little information. The concentration of zircon can 

 of course be readily ^ justified on the grounds of its hardness, 

 lack of parting or cleavage, chemical resistance and its rather 

 high specific gravity (4*7). But as to the resistance of mag- 

 netite, I have not been able to find any data. It has no 

 appreciable cleavage and is often free from parting, in which case 

 it may be assumed to be mechanically resistant, though it is not 

 very hard. As to its chemical resistance I can find no evi- 

 dence, but its familiar prominence in many stream sands and 

 above all in beach sands, and the frequency with which it 

 forms an important constituent (between 15 and 20 per cent 

 by weight of the heavy minerals) in sands of the Upper Cre- 

 taceous of Maryland that I have studied (believed to cover a 

 range of types from delta to true marine sediments), justifies 

 the conclusion that it is chemically resistant to atmospheric and 

 aqueous agents of decomposition, at least in temperate regions. 



Since the balance of the minerals, so very scantily present, 

 are all more or less regular constituents of sedimentary rocks 

 and are not, in this case, the product of any one kind of 

 igneous or metamorphic rock, they contribute nothing of 

 especial use for this discussion. But they do furnish certain 

 negative evidence which is important, namely the striking 

 scarcity of epidote, and the apparent absence of chlorite, two of 

 the most common weathering products of crystalline rocks, 

 and therefore, as a consequence of their wide distribution and 

 chemical stability, two of the most persistent and often abun- 

 dant constituents ,of sedimentary rocks. The absence of 

 chlorite, however, must be grouped, from another point of 

 view, with the absence of micas in general. Finally as a third 

 negative character is the practical absence of ferromagnesian 

 minerals, though their stability not being as great as that of 

 epidote and chlorite they are under no conditions as likely to 

 survive. 



This evidence when combined points to the following con- 

 clusions. The scarcity of epidote and absence of chlorite 

 indicate fresh source rocks and mechanical — that is arid or 

 glacial — as against chemical disintegration. But the great 

 scarcity of micas and of ferromagnesian minerals are two 

 results distinctly characteristic of arid conditions alone. Good- 

 child* explains the elimination of ferromagnesian minerals 

 under arid conditions as resulting from their pronounced 

 cleavages which tend to cause them to splinter up under the 

 action of extreme temperature changes, so that they are readily 



*Goodchild, J. G. : Desert conditions in Britain, Trans. Geol. Soc. Edin- 

 burgh, vii, p. 206, 1896. 



