432 Scientific Geology. 



original rock be essential to its production. This seems rather to 

 have formed the paste in which are cemented dark pieces of trap. 

 This rock extends but a short distance upwards from the line of junc- 

 tion. 



The most interesting effects at this spot are exhibited, however, in 

 the sandstone beneath the trap. Like that near Hartford, for two or 

 three feet its schistose structure is in a great measure obliterated, al- 

 though its stratification remains. It is also of a light gray colour. 

 On breakingit, it exhibits a semi-crystalline structure, bearing consid- 

 erable resemblance to some varieties of fine grained granite. (No. 

 170.) But the most curious fact of all is, that this rock exhibits in 

 some places a decided tendency to a columnar form. I cannot say, 

 indeed, that any perfectly formed prism can be found. Still the 

 sandstone exhibits several unequal sides of a prism, perpendicu- 

 lar to the planes of stratification, and thus coinciding with the less per- 

 fect columnar structure of the greenstone immediately above: so that 

 at a little distance one does not perceive the line of junction between 

 the two rocks. 



I have met with no geological writer but Dr. Macculloch, who has 

 described examples of columnar sandstone ; and he mentions but two 

 cases in nature, both in Scotland. Another case, however, is noticed 

 by him in the hearth stone of a blast furnace.* This last example is 

 very instructive, as showing that heat, if long continued, may produce 

 the columnar structure in sandstone even though not intense enough to 

 melt it. The application of this case in explanation of the columnar 

 structure of the sandstone at Titan's Pier, is too obvious to render its 

 formal statement necessary. 



Some of the sandstone of the new red sandstone is highly micace- 

 ous : this variety, where it occurs near the greenstone, can hardly be 

 distinguished from mica slate. (Nos. 177 to 179.) I will refer only to 

 two localities of this rock, viz. at Turner's Falls and at the north end 

 of Mount Tom, at the spot where the sketch on page 429 exhibiting the 

 junction of the two rocks, was taken. Can there be any doubt that 

 these examples are in fact a partial conversion of the sandstone into 

 mica slate by the heat of the greenstone ? 



I know of but one place in the valley of the Connecticut where 

 greenstone comes in contact with limestone; and that is in West 

 Springfield. Perhaps even there an actual contact does not exist, yet 



* System of Geology, vol, 1. p. 172. 



