328 F, RUTLEY ON COMMUNITY OF STRUCTURE 
For example, the terms dolerite, anamesite, and basalt are ac- 
cepted as indicative of variation in the texture of certain rocks 
which are identical in mineral constitution. We may apply any 
of these terms to such a rock, but we do not imply auy difference 
either in the nature or in the relative amounts of its mineral consti- 
tuents. 
These names simply indicate varieties of texture or crystalline 
development; and they probably only denote differences in the 
temperature at which the rock was erupted, or differences in the 
rate of cooling during the solidification of any such mass of rock. 
Be these differences great or small, the origin of these rocks has 
been approximately the same. The temperature during eruption 
and the rapidity of cooling are simply subordinate and relative 
questions: they are mere questions of degree; and dolerite, ana- 
mesite, and basalt are mere phases of one and the same rock. 
Very fine-grained or cryptocrystalline rocks of the basalt group, 
the diorite group, &c. are designated aphanite, a good and useful 
term under which to cloak ignorance; and aphanite, in part, may 
be regarded as a still lower grade in the crystalline development of 
a basalt. 
The same may be said, structurally, of the granitic rocks, which 
range from the coarsely porphyritic granites to the fine- grained 
élyans. 
Between the extremes of development of the constituents in such 
rocks there lies much the same difference as that which exists 
between a dwarf and a giant. The principle which governs the 
development of the large crystals is also evident in the structure of 
the smaller ones. 
If, then, in our classification of rocks such questions of relative 
magnitude i in the development of crystals may be ignored, how much 
more justly may they be set aside when we come to consider the 
structure of clastic rocks? The differences between shingle, scree- 
material, gravel, and fine sand merely represent the differences which 
we find in their cemented and consolidated representatives, conglo- 
merates, breccias, grits, and sandstones. ‘The finest-grained sand- 
stone or grit is but a conglomerate or breccia in miniature. The 
terms lapilli and ash, as applied to volcanic ejectamenta, also merely 
indicate degrees of comminution. 
Certain substances, however, are more easily susceptible of fine 
division than others ; and in the mud which, on consolidating, forms 
shales and slates, aud in the more finely pulverized volcanic dust 
which on levigation gives rise to mud streams, such as the moja of 
the Andes, we find probably the extreme limits of mechanical 
power as applied to the natural disintegration of rock matter. 
Before quitting the subject of relative dimensions in the constituents 
of rock masses, it may be worthy of remark that the term porphyritic 
is one which is conyeniently, though unjustly, limited. Rocks which, 
to the naked eye, present no porphyritic appearance whatever are 
often seen, under the microscope, to be perfectly deserving of the title, 
the magnified image exhibiting all the characters which distinguish 
