232 



unifonnly crystalline, massive diabase, the texture being pro- 

 portional to the magnitude of the dike. Another argunient 

 for the dike is that the strata on o[)po8ite sides of this line of 

 outcrops are, as the map shows, clearly and entirely at variance 

 in dip and strike, and a fault coinciding with the dike is a 

 structural necessity. Whether the dike actually ends against 

 the melaphyr as represented, it is impossible to determine, 

 since it passes in this direction beneath an extensive wet 

 meadow and outcrops are wholly wanting. The coarse, 

 holocrystalline, and homogeneous character of the diabase 

 utterly forbids connecting the dike with the melaphyr as a 

 possible channel of supply for the latter. The melaphyr is 

 certainly older and the dike is just as clearly newer than the 

 bordering sedimentary rocks ; and the smaller dikes running- 

 south through the sandstone and conglomerate are probably, as 

 indicated on the map, branches of the main dike. 



The strata between the great dike and the melaphyr of 

 Downer Avenue and Lincoln Street form, apparently, a low 

 monocline of 15° — 30°. We conunence on the avenue, north 

 of Planter's Fields Lane, with outcrops of a purplish and gray 

 slate passing south into gray sandstone with fine pebbly 

 layers, the whole dipping S. 30°. The sandstone can be 

 traced west across the fields, as mapped, and is seen to change 

 gradually upward into the small-pebbled conglomerate which 

 outcrops so prominently along the entire distance, especially 

 west of the lane. The dip of the conglomerate was not 

 clearly observed ; but it passes on the south, through sandstone 

 to a finely banded slate, 75 feet in breadth, with a southerly 

 dip of only 15°. South of the slate are several sligiit 

 exposures of sandstone before we come to the conglomerate 

 crossed by the lane, near the melaphyr. This outcrop is more 

 extensive than marked, extending 200 feet southwest (;f the 

 lane. 



The correlation of these beds is not easy. Their surface 

 exposures or developments, as compared with other sections. 



