494 GEOLOGY OF OLD HAMPSHIRE COUNTY, MASS. 



associated together as of the same age and contrasted with the two latter 

 and older occuiTences ; but olivine occurs in considerable abundance in the 

 base of the Holyoke bed at the west foot of Provens Mountain in Westfield. 

 It is strange, however, that where the inclusions of quartz are most abun- 

 dant olivine is also most abundant. 



NORTHAMPTON. 



THE TWELFTH OK BURNT MILL CORE. 



Beneath the railroad bridge over the brook which crosses the railroad 

 a mile above Smiths Ferry the contact of the sandstone dips steeply south- 

 east beneath the posterior trap, and this contact can be followed, clearly 

 exposed, along the south side of the brook beneath the road bridge and 

 past the ruined leather mill, and along the south side of the mill pond. 

 The conformable posterior trap bed extends southeast of this line. (See 

 p. 465.) Just north of the brook and below the dam one sees the outcrops 

 of an intrusive dike or plug, which comes up apparently along a transverse 

 fault that can be traced west across the mountain from this point. The trap 

 cuts across the strata of sandstone that underlie the posterior trap bed at 

 every angle, and adjacent to the north end of the dam bakes the sandstone 

 for 6 or 8 feet, so that the line of contact between trap and sandstone is 

 seen with difficiilty. 



A long ridge of the trap extends west along the northwest side of the 

 pond, and appears also on its south side, and can be followed thence south 

 for 20 rods. At the water's edge trap and sand are confusedly blended. 

 In the bluif above, just south of the head of the pond, appears a well-marked 

 fault. The posterior trap abuts on the sandstone. It is the continuation of 

 the Mount Tom fault, and the newer trap comes up at the intersection of 

 the two faults. A lai'ge part of the trap is fresh, compact, and breaks with 

 sharp splintery fracture; a portion of the surface is crumbly and much 

 weathered. This represents a part of the surface of the old laccolith. 



The trap of the plug may be followed down the brook on its north 

 side to the road, and just east of the road several offshoots from it appear on 

 both sides of the brook. On tlae north is a 4-foot dike, on the south a 4-inch 

 dike at the water's edge and a 1-foot dike a few feet up. The latter con- 

 tinues under the road to the dam. A few rods farther east one comes 

 upon the most southern of two outcrops of trap at the Lyman raih'oad 

 crossing, which is doubtless the eastern edge of the plug, since it is com- 



