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it is very certain that their non-occurrence over the broad area 
of melaphyr forming Atlantic and Centre Hills cannot be 
attributed to the absence of exposures. Only three dikes in this 
belt (3—5) belong to the first or oldest system ; while no fewer 
than fourteen (6-19) find a place in the second system. The 
older dikes will be described first and, so far as possible, in the 
topographie order of the outerops. 
On the northeast corner of Atlantic Hill, the melaphyr south 
of the great dike (7) is, for a breadth of nearly twenty feet, 
very shaly, being divided into thin vertieal layers running N. 
80° E. Careful examination discovers in the middle of this 
shaly zone a dike (3) having the same trend. It must meet 
the great dike near the east side of Valley Beach ; and is prob- 
ably eut and slipped by it, although the intersection is not 
exposed; while to the westward it passes under the shingle 
beach. It is finely porphyritic, and reappears with the same 
lithological character, width, and hade on Gun Rock and the 
half-tide ledge to the eastward. 
From 25 to 75 feet south of the melaphyr dike (1) on 
Centre Hill, and extending from near the end of Centre Hill 
Avenue across the ledges to the shore, is the narrow chasm 
holding the very obscure outcrops of the next dike (4), the atti- 
tude and dimensions of which are taken from the fissure rather 
than from the dike itself. It trends toward the middle of Gun 
Rock, but no traces of it could be found on that promontory. 
Nearly 150 feet south of this dike is a broader chasm or de- 
pression, in which a small dike occurs, or possibly in the lower 
end of the chasm two small parallel dikes with several feet of 
melaphyr between them. Looking eastward across the bay we 
see in line with this depression a more sharply defined chasm in 
the first ledge south of Gun Rock. Here the double dike is 
well developed in a fissure five feet wide, the south dike meas- 
uring 2.5 feet and the north dike 1 foot. Keeping this course, 
we find the same double dike cutting the ledge east of Gun 
Rock. The two members here measure 3 and 1.5 feet respec- 
