THE LONGMEADOW SANDSTONE. 367 



twinned crystals .03 to 04""" long, maxiinuin extinction 17''. Wlien the 

 calcite is dissolved they ajjpear perfectly limi)id and fresh, and often 

 show the marked nndnlatory extinction characteristic of aqneous albite. 

 One of the larger feldspars of first consolidation, cut })aralled to M (010), 

 showed the optical figure almost central, with the axial plane at 103° to the 

 vertical axis of the crystal, thus having all the characteristics of all)ite. 

 The smaller feldspars are more basic. All the constituent feldspars are 

 dusted with kaolin, but their properties can be made out clearly. The rock 

 may have been bleacOied somewhat by acid waters and the iron may l)e 

 now present in the })yrite, but when the calcite is dissolved with acid the 

 rock seems little decomposed, and it is of the same character throughout 

 the compact mass from sui'face to center. 



It must, therefore, have differed materially from the Cheapside trap 

 when fresh, although more like it than any other variety in the Trias. 

 The large amount of calcite in the bed indicates a considerable body of 

 lava as its origin. The wholly angular character of the fragments Avas due 

 to an explosive eruption not far distant. I have little doubt that the focus 

 of this erudition is to the east, along the old main fissure, concealed l)y the 

 masses of the Forest Park plug and the newer sandstones. 



DISTURBANCES IN THE SANDSTONES AND INCLUSIONS OF TRAP P'RAGMENTS JUST 

 BELOW THE POSTERIOR SHEET. 



Where the western lobe of sandstone passes down between the areas 

 of trap which extend north from the region of the Little Mountain core the 

 dark-gray sandstone, at a horizon just below the posterior sheet, is much 

 contorted, bands of the sandstone being twisted into sharp zigzags in a few 

 inches. It makes the impression of some local disturbing force acting before 

 the hardening of the sand to rock, like the "wallows" in the sandstone at 

 Turners Falls, formed by the crowded tracks of the great reptiles. At 

 other localities along the western ])art of the eastern lobe of sandstone 

 which projects into the trap area this disturbed layer is covered by sev- 

 eral feet of undisturbed, flat-bedded sandstone, the disturbed sandstone 

 graduating rapidly into the undisturbed rock and showing the contortion to 

 have been produced before the deposition of the latter. There are a few 

 small anticlines in the rocks at this place which are easily distinguished 

 from the structure in question. The band is the more interesting because 

 it contains angular fragments, 6 inches long, of the common black trap. 



