284 REPORT OF NATIONAL MUSEUM, 1904. 



of trap. "This hill," he wrote, "exhibits that gentle acclivity and 

 rounded summit so common in the transition formations of the Wer- 

 nerian school.' 1 



The Medford trap (diabase) he noted as being- unfit for architectural 

 purposes, owing to its rapid disintegration, a fact which has been 

 many times commented upon in more recent years. The tendency 

 manifested by the "greenstone" as a whole to exfoliate in bowlder 

 forms with concentric structure, he correctly ascribed to weathering, 

 as did Gibson writing several years earlier (p. 254). 



lie noted the occurrence of abundant joints in the conglomerate of 

 Dorchester, but considered such as inexplicable with the geological 

 information then available. An interesting light is thrown upon the 

 lack of knowledge of the chemical composition of rocks at that time 

 in the continuation of his paper the following year, in which he 

 described this conglomerate — a highly siliceous rock — as passing into 

 the overlying melaphyr, a basic igneous rock. In several places 

 within the town of Brighton he thought to note the transition from 

 one rock type to another." 



In the American Journal of Science for 1827 Alanson Nash pre- 

 sented his first, last, and only geological paper that seems to have 

 found its way into print — this relating to the lead mines and veins of 

 Hampshire County, Massachusetts — and offered some 

 Firm S ati<^ as i h 8°27 Vein interesting speculations as to their origin. 



That the}^ were not once open fissures tilled from 

 above, the fissures themselves being formed by the unequal subsidence 

 of the earth's crust or through shrinkage caused by desiccation, accord • 

 ing to the Neptunian theory, was to him evident for the following 

 reasons: If the cavities were formed by desiccation and subsidence the 

 veins would be widest at the surface and narrow as they descend, 

 whereas, in fact, the very reverse is the case. If filled from above by 

 mineral solutions which covered the globe, then he thought we ought 

 to find beds of metallic matter in the valleys and plains also. Neither 

 was he disposed to accept the views of the Plutonists, who regarded 

 the veins as filled lyy "an injection from a fiery furnace below." Rather 

 would he look upon them as contemporaneous both in formation and 

 tilling with the rocks in which they occur, being analogous to the 

 granite veins of the same region. That the vein material did not 

 adhere firmly to the wall rock, as is the case with the granite veins, 



"This deceptive appearance, it may be stated, is due to the fact that the melaphyr 

 at the time of its extrusion was in a highly liquid condition and flowing out over the 

 uneven surface of the conglomerate rilled in all the inequalities, so closely welding 

 itself as to form what was apparently one and the same mass. When subsequent 

 erosion cut away a considerable portion of both rocks the appearance of isolated 

 patches of melaphyr here and there on the eroded surface of the conglomerate was 

 quite misleading. Some more recent observers have since committed the same 

 blunder of observation and faulty deduction with far less excuse. 



