1^2 MINERALOGY AND LITHOLOGY. 



pie : 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. 



