132 BULLETIN OF THE 



the dike and sandstone is sometimes blurred, as if they had been lo- 

 cally melted together ; and the texture of the dike becomes finer on ap- 

 proaching close to the included sandstone fragments, just as it does on 

 approaching its sandstone walls. 



Returning to the quarry, we find that the sandstone from the breccia 

 bands has no indication of induration, except tbat resulting from the 

 moderate cementation of its clastic material by secondary quartz and 

 calcite deposited around the grains. Sections of the sandstone in con- 

 tact with the included trap fragments and with the main mass of the 

 trap sheet show a well marked laminated arrangement of the sand grains 

 Jiearly parallel to the walls of trap and to the faces of the trap frag- 

 ments ; this points decisively to the deposition of the sandstone poste- 

 rior to the eruption and Assuring of the trap. There is also a laminated 

 arrangement of the sand grains on all sides of the trap fragments, as far 

 as examined, which we do not fully understand, but which may be per- 

 haps interpreted as indicating continued motion of the faulted masses 

 while the breccia was still moist and soft, every trap fragment moving 

 as a whole and thus calling for an adjustment of the sand grains around 

 it. There is no change in the texture of the enclosing mass of trap on 

 approaching the breccia bands, such as would certainly appear if the 

 sandstones were inclusions. A change of texture is so characteristic of 

 rapid marginal cooling that it is often shown immediately at the bor- 

 der of large amygdaloidal cavities, as has been mentioned by Pumpelly,^ 

 and as is well marked in our slide 141, from near the upper surface of 

 the lower trap in the Meriden quarry, and again still better in slide 

 218 from the Middlefield Railroad cut, locality 22, in which a nearly 

 spheroidal vesicle is surrounded by a layer of trichitic glass having an 

 area as large as the vesicle itself. 



In order to apply this test carefully to the case in hand, several sec- 

 tions were cut from the trap in the quarry, on either side of the best ex- 

 posed breccia, at the contact, and one and four feet away. These show 

 no tendency towards a finer grain, or towards a development of porphy- 

 ritic crystals or glassy character on nearing the breccia ; the character 

 of the trap remains constant to the contact. Moreover, the angular 

 fragments of trap in the breccia are of uniform texture, and are identical 

 with the trap on either side, except for a little greater weathering in the 

 former. These fragments may therefore have been derived by fracture 

 directly from the enclosing walls ; but certain minute grains of very 

 fine-grained decomposed trap, also occurring in the breccia, appear to 



^ Metasomatic Development of the Copper-Bearing Rocks, loc. cit., p. 283. 



