Notes on Monhegan Island. — Lord. 347 
beerbachyte (I) and malchyte (II) have essentially the same 
composition. The mean of these two analyses (VI) is seen, to 
correspond almost exactly with that of gabbro-dioryte and 
hornblende-gabbro (V) — the only appreciable difference being 
in MgO, FeO, which is 2.53% larger in the dike rocks (VI) 
than in the gabbro and gabbrodioryte (V). 
From this striking chemical as well as mineralogical anal- 
ogy it is apparent that these dikes represent but another form 
of the same gabbroitic magma which, fluctuating within the 
limits of columns III and IV, has itself reached an advanced 
stage of differentiation from that of the common magma. (See 
Analysis I p. 340.) 
It is also probable judging by the intimate geological as 
well as mineralogical relations of the main rock types that this 
differentiation was caused by slight local changes in the physi- 
cal and chemical conditions during the solidification of the 
liquid mass after it had reached essentially its present geolog- 
ical position. The presence of numerous dikes, exhibiting the 
same mineralogical variations as occurred in the main rock 
mass itself, substantiates this view, and points, furthermore, to 
the very gradual and uniform character of the magmatic fluc- 
tuations. 
Monhegan has participated in the principal orographic 
and epirogenetic movements of the adjoining province, and 
may well be considered a part of it(see p. 329), although rocks 
of similar description were not found on the neighboring 
islands, nor in the immediate vicinity on the mainland. 
Owing to the isolated position of the island and the ab- 
sence of sedimentary rocks upon it, but little more can be said 
of its geologic age than that the intrusion antedated that of the 
granitic dikes. 
Very similar conditions are found on the island of Vinal 
Haven* where the diabasic gabbro, (black granite), of post- 
Niagara age is intersected by younger granitic intrusions. If 
this diabase is indeed a geological equivalent of the Monhegan 
rock, which on purely petrographic grounds seeems probable, 
both intrusions would be of late Devonian age. 
*See G. O. Smith, op. cit., p. 64-65. 
