Petrographical Differentiation of certain dykes. 159 
more than a classification of hand specimens ? Of what philo- 
sophic or geological value is a classification of specimens into 
different species and types when the}' ma}- all be one and the same 
rock crystallized from the same magma within a few feet of one 
another. The geologist who knows his rocks in the field as well 
as in the laboratory finds such classifications very little expressive 
of geological truth. But the differentiation in character of this 
dyke rock suggests other matters than a criticism of classification, 
namely, a consideration of the conditions under which such differ- 
entiation was developed from a common magma. From the nature 
of the case we are able to form fairly satisfactoiy conceptions as 
to two conditions which are commonly regarded as having a para- 
Fig. 3. 
Stop Island dyke. — Section of dyke-rock seventy-five feet from dyke 
wall (middle of dyke). X38. f. plagioclase; h. hornblende; q. quartz; 
m. magnetite. Illustrating allotriomorphic-granular or granitic 
structure. 
mount influence upon the solidification of rock from magma. 
These are (1) the relative pressure, and (2) the relative rate of 
cooling under which the different parts of the magma solidified. 
The magma being confined between parallel sharply-cut fissure 
walls may fairly be assumed to have; been under the same con- 
stant hydrostatic pressure in any given horizontal plane during the 
