172 MINERALOGY AND LITHOLOGY. 
ple: the diabase of the Connecticut valley, when in large dykes, usually 
possesses a more coarsely crystalline structure than when in little ones, 
and the sides of the dykes which are in contact with the country rock 
are finer in texture than the centres. These differences in texture are 
produced by the influence of the surrounding rocks on the rapidity of 
cooling. Again: columnar structure, which, both in shape, size, and 
direction of the columns, is dependent upon certain well understood laws 
of cooling, is everywhere apparent. Our old trap rocks show but little 
of all this. The smallest dykes are often coarsest in texture, and hence 
we must conclude that either these rocks were erupted into cracks in 
hot rocks, that they did not reach the surface, and hence solidified under 
pressure, or that they have, with the strata in which they occur, been 
subjected to metamorphic action subsequent to their eruption. As any 
or all of these causes may have acted to produce structural differences, 
we can understand why our basic eruptive rocks are so diversified; why 
diabase, diorite, and gabbro were formed out of nearly the same ma- 
terial; and why coarse or fine, compact or porphyritic rocks were made 
in fissures of the same form. Moreover, it has already been pointed out 
that movements took place in the half-made rock, breaking up crystals 
already formed, and apparently introducing new conditions for finishing 
the solidification. In the gabbros, the well formed crystals of apatite, 
which after being perfectly formed were again partially dissolved, point 
also to variations in condition. The consideration of this element of 
variable conditions during the solidification of the rocks accounts for 
many differences in texture, and may especially be applied in the con- 
sideration of the porphyritic varieties. 
In reference to alteration and decay, it might appear that enough had 
been said, since the description of the rocks has consisted largely in 
details of the modes of decomposition. The natural conclusion of the 
process remains, however, to be considered. In New Hampshire one 
often finds dykes of compact white or light yellow material. This is 
usually one of two kinds of rock. Either it is a felsite, or it is a diabase, 
which by decomposition has lost all resemblance to its original self. 
Often this material can be found in connection with less modified por- 
tions, and the stages of decay can be noticed. I have sections cut from 
white decomposed diabase from Bemis brook and the Lincoln flume. 
