474 Scientific Intelligence. 



represent a subaqueous delta of the Potomac river, formed when 

 the sea rose far above its present level and fashioned the marine 

 terraces exhibited in the bluffs. Its absence above sea level east 

 of the Eastern Branch may be attributed to a dislocation (of 

 which there is other evidence) trending parallel with the Appa- 

 lachians and the Atlantic coast. The most extensive formation 

 in the District is that hitherto known as " Newer Mesozoic" in 

 Virginia, and "Iron Ore Clays" in Maryland. It is denominated 

 the Potomac formation. In structure and composition it is bipar- 

 tite, the upper portion consisting of highly colored banded and 

 mottled clays, with intercalations of sand and quartzose gravel, 

 and the lower of sand and gravel with intercalations of clay. In 

 both divisions stratification is inconstant and often absent, and 

 the materials are sometimes indiscriminately intermingled. The 

 formation is practically destitute of fossils in the District, but 

 yields abundant plant-remains in Maryland and Virginia. It 

 appears to consist of inosculating deltas of the Potomac and 

 other Atlantic coast rivers and the littoral deposits into which they 

 merge, laid down along a bay-indented coast upon a highly 

 inclined and irregular sea-bottom produced by combined depres- 

 sion and sea-ward tilting of a deeply corroded land surface in late 

 Jurassic or early Cretaceous time. The Potomac formation rests 

 everywhere on the eroded edges of highly inclined gneiss which 

 has not yet been thoroughly studied, but which is probably an 

 extension of that of New York and Philadelphia. 



W. J. McG. 



3. Theories of Ore-deposits • by M. E. Wadsworth. (Proc. 

 Boston Soc. N. Hist., xxiii, 197.) — Professor Wadsworth, after 

 referring to a tendency to change in the minerals of igneous and 

 metamorphic rocks, and speaking of the unstable condition as a 

 consequence of origination at high temperatures, explains the 

 changes by appealing to "percolating waters" and "external 

 agencies," and refers to the same causes the "segregation or 

 localization of ores " into veins, where such veins are not directly 

 of igneous origin. His closing paragraph (p. 203) is as follows : 

 "The general alteration manifests itself in a universal chemical 

 or molecular transference — a transference of material, leading to 

 the segregation or localization of (he ores in the places in which 

 they are now found ; it manifests itself in the deposition of min- 

 eral matter in the veins and cavities of the rocks themselves, in 

 deposits from springs in bogs, lakes, etc. From this it would 

 follow that all ore-deposits not eruptive are superficial phenomena 

 as regards the earth and dependent on its external agencies, 

 although they may be deep enough so far as man is concerned. 

 Ao-ain, few of these ore-deposits would be expected except in 

 regions in which percolating waters and their resulting metamor- 

 phism have been efficient agents; while the various forms of ore- 

 deposits would be expected to be associated with and grade into 

 one another." 



The method urged appears to exclude any aid in the making of 

 veins from vapors of water or other material ascending from the 



