342 The American Geologist. December, i9uo 
Residue: .83% CaO+.49% ignition+.4i% (1.66 TiO^ less 
equivalent 810^=1.73%. 
Acid and Basic Dikes of the Region. 
As has already been stated (p.332)fissure instrusions are of 
common occurrence in the Monhegan mass. They are indeed 
so numerous that it was not attempted to represent all on the 
map. These dikes belong to two great systems of intrusions, 
differing in time of eruption, as well as in mineral composition. 
To one system belong all the basic dikes closely allied geologi- 
cally and mineralogically with the basic rock of the region; 
and to the other the granitic and aplitic types belonging to a 
later period of eruption and in no way connected genetically 
with the Monhegan mass. It is of interest to note that the basic 
dikes are confined almost exclusively to the island, and have 
little or no uniformity in strike; whereas those of granitic 
character, with but one or two exceptions, extend uninterrupt- 
edly in a N. 50°-6o'*'W. direction across the island, and could 
be identified again on the main land some twelve miles distant. 
These acid dikes vary in width from 6 inches to as many feet, 
and in texture from a fine grained soda-aplyte composed of al- 
bite, muscovite, quartz and accessory garnet, cordierite and 
magnetite, to a coarse granular pegmatitic granite made up 
principahy of albite> quartz > muscovite > orthoclase> gar- 
neta> apatite > magnetite. 
The exceptions referred to are of two dikes from 6 in. to 
a foot in thickness outcropping along the eastern face of Burnt 
Head, and continuing in a northerly direction to White Head; 
a distance of about Yz mile. The rock is characterized by the 
presence of biotite and blue and black tourmaline graphically 
intergrown with quartz. 
The texture of these dikes is usually quite uniform, but in 
some instances rocks of granitic and aplitic character occur 
within the same vein. Here the aplyte occupies the central 
part of the dike mass in such a manner as to suggest a later 
intrusion. The results of a chemical analysis of this aplyte 
(No. 17) is given under I, and under H that of a similar rock 
