430 



Scientific Geology. 



mile long-, at Rocky Hill, about three miles southwest of Hartford. 

 The trap is here superincumbent upon the sandstone, and this latter 

 rock is changed to the depth of about four feet below the junction. 

 Ascending from that depth, it begins to grow firmer ; the color grows 

 lighter, the red vanishes and it becomes dark gray — light gray — ash 

 gray, and in some places almost white ; while at the same time the 

 firmness is much increased, so that from being a very soft and tender 

 argillaceous sandstone, easily splitting into laminae, it has become 

 hard, and difficult to break, striking fire with steel like an overburnt 

 brick, and its fissile character is almost or quite destroyed." 



" But this is not all. At the depth of about two feet, rather less 

 than more, the altered sandstone begins to grow vesicular. Fine 

 pin-hole cavities make their appearance; they are very numerous, and 

 the solid substance which surrounds them becomes semi-vitreous and 

 loses the appearance of sedimentary or fragmentary matter ; as we 

 ascend towards the trap the vesicles increase rapidly in size, and at 

 and near the junction they are both numerous and large.' 3 



This vesicular structure is still more remarkable in the trap, ex- 

 tending several feet upwards ; and near their junction, the two rocks 

 can hardly be distinguished, and appear as if melted together. 



Similar phenomena, more or less strongly marked, present them- 

 selves both in Connecticut and Massachusetts, where the contact of the 

 iwo rocks is visible. On the east side of Mount Tom in Northampton, 

 and on the south side of Holyoke, the vesicular character of the sand- 

 stone is most obvious : as is that also of the greenstone. (No. 286.) 

 From the description that has been given of the relative position of 

 these rocks in those places, it will be recollected that the sandstone is 

 uppermost. The cavities are sometimes rilled with some mineral, as 

 carbonate of lime, subsequently introduced ; but the red color of the 

 rock is generally retained : sometimes, however, it is not easy to dis- 

 tinguish this amygdaloidal sandstone from trap, without close inspec- 

 tion. Yet in most cases the line of junction is distinct, and the schis- 

 tose structure of the sandstone is not lost. The greenstone, as already 

 mentioned, is in these instances much more vesicular than the sand- 

 stone, and to an unknown depth. The cavernous base, the cavities 

 not being usually filled, differs but little from indurated clay ; and 

 some circumstances have led me to suspect that the rock in fact con- 

 sists of argillaceous sandstone or shale, which has been fused. 



A little below Turner's Falls on the Greenfield shore, the junction 

 of these rocks may be advantageously examined, where they occupy 



