report of The director and state geologist 1901 r23 



dence that the trap came up through the Silurian and subjacent 

 terrane, as held in this paper, and that the rock is not to be re- 

 garded as an in-faulted remnant of a lava flow once covering the 

 Hudson terrane in this vicinity. 



The occurrence of these inclusions surrounded by the lava 

 crusts throws some light on the origin of the ball structure. 

 These foreign fragments having absorbed edges would doubtless 

 rob the surrounding lava of heat and thus become centers of 

 cooling. The lava next to the inclusions, thus robbed of some 

 of its heat and further altered in composition by the absorption 

 of lime carbonate, might cool more quickly and with a different 

 structure from that of the ground mass. In this view the lava 

 crusts are tachylitic variations of the magma both about the 

 limestone inclusions and about the cindery nodules. The pum- 

 iceous nodules, resembling the centers of many so called bombs, 

 evidently may arise from the occlusion and segregation of gases 

 in the lava. The explosive expansion of these gases would pro- 

 duce a sudden lowering of temperature, however slight, even 

 under the conditions of a hot magma, possibly sufficient to 

 determine the initiation of cooling in the contiguous lava, and 

 thus would, in conjunction with the chemical reaction on the 

 lava, lead to the development of a tachylitic zone in the basalt 

 about the gas-charged, vescicular lava. This effect would arise 

 as well from slaked limestone inclusions as from simple gas- 

 charged lavas. In the case of the limestone, the gas would be 

 derived from the absorbed portion of the rock. 



The surrounding ground mass with a flowage structure, in 

 which such lava balls are commonly embedded, evidently owes 

 its structure to later and more rapid cooling. 



The fine grained ground mass with fissile structure is much 

 disintegrated at Stark's Knob. It appears to the eye in many 

 places to be clayey or sandy, often weathering to a black, sandy 

 mud. It is frequently ramified by small calcite veins. Where 

 fresh exposures occur, the rock has a resinous luster and is 

 here taken to be a diabase pitchstone. It has not been studied 

 in thin sections. 



