1G0 The American Geologist. March, isoi 
time of its solidification. It may also be fairly assumed that 
those portions of the magma adjacent to the fissure walls cooled 
more rapidly and solidified earlier than did the middle portion. 
Thus although we have in current petrographical literature numer- 
ous references to great pressure as one of the chief causes of the 
development of the coarser texture and granitic structure of the 
plutonic rocks as compared with that manifested by rocks which 
solidify at or near the surface, the present case seems to establish 
the fact that both types of rock structure may be developed under 
one and the same pressure. Difference in pressure under which 
magmas solidify is, therefore, probably, not so important a cause 
of the difference in structure and texture of rocks as is generally 
supposed. On the other hand we have in the case under con- 
sideration strong presumptive evidence that the rate of cooling 
which must have been rapid at the sides and slow in the middle, 
exercised the controlling influence over the character of the rock 
developed from the magma in any given part. With regard to the 
conditions which determined the chemical and mineralogical differ- 
entiation of the dyke rock very little can be definitely affirmed. 
It seems probable, however, that the explanation lies in the earlier 
separation of the more basic minerals accompanied by a trans- 
ference of more acid residues (or solvents) to the middle portions, 
which transference was facilitated by the gradual solidification of 
the magma from the dyke walls toward the middle, and by the 
movement of the water constituent of the magma towards the 
middle. The water of the magma, so long as the latter remained 
liquid, would have a tendency to escape to the surface. This 
tendency, taken with the tendency of the higher portions of the 
dyke to solidify more rapidly than the deeper-seated portions 
would create a current obliquely through the magma, upward and 
inward from both sides. This current would aid in the transfer- 
ence from the sides to the middle of the more acid portions of 
the magma from which the more basic had separated out. 
Numerous other d}'kes have been examined with the same gen- 
eral result as that arrived at by a study of Stop Island dyke. In 
none of these, however, was the differentiation in character found 
to be quite so strongly accentuated as in the Stop Island dyke. 
A series of specimens taken across the dyke which cuts the south- 
east shore of White-fish bay, and which is referred to in former 
