26 
The arrows pointing. away from the fault lines on the map 
indicate the direction of the downthrow in each case; and the 
accompanying figures, when present, express the amount of 
the displacement in feet wherever it could be even approxi- 
mately determined. 
The map also brings out very clearly the principal facts con- 
cerning the dikes of Nantasket — their sizes, trends, distribu- 
tion and correlation in different outerops. Mr. Merrill has 
shown that they consist almost exclusively of diabase, and are 
all highly altered; although they exhibit some notable differ- 
ences in texture and degrees of alteration. The dikes are 
numbered on the special map, and the numbers are repeated 
for each important outcrop, so as to express more clearly their 
individual continuity. The most of the dikes are readily arranged 
by their trends in three distinet systems. The dominant trend is 
manifestly approximately east-west. But these are seen on closer 
inspection to embrace two systems, diverging 20° to 80°. The 
normal trend of the first or oldest system is N. 75° to 80° Му, 
and of the second S. 75? to 80° E. Their relative ages are in- 
dicated by a slight difference in texture and especially by a very 
clear intersection on the shore east of Gun Rock and several inter- 
sections on East Porphyrite Hill. The third system trends 
due north-south and is the newest of all, as shown by two 
intersections of dikes of the second system : Dike 62 cuts dikes 
13, 14 and 15 of the second system in Green Hill Ledge very 
clearly, indeed; and the composite dike (66) in the gorge 
between East Porphyrite Hill and Cliff Plateau certainly cuts 
31 of the second system which in turn cuts 22 and 24 of the 
first system on East Porphyrite Hill. Notwithstanding the 
regularity of their forms and trends, the dikes, with the partial 
exception of the newest system, exhibit a remarkable independ- 
ence of the best developed systems of joint-planes. This is 
especially noticeable along the northern bases of Atlantic and 
Centre Hills, indicating that the older dikes, at least, probably 
antedate the joint-structure of the rocks to some extent. А 
further inspection of the map will show, however, that' not only 
