.IGNEOUS FORMATIONS. 303 



to explain the fact that while the magma of the dike and masses under 

 discussion consolidated at perhaps 1,000 feel from the surface, its surface 

 lavas are now seen at approximately the same horizon. The conditions of 

 cooling and final crystallization of these masses were nearly the same as 

 for the Vahnont nick, and the product is naturally a similar one. 



It is probable that the < iretaceous shales traversed by these dikes were 

 much fractured l>v the folding between the Laramie and Arapahoe epochs, 

 and that t<> this fact is due the large number of small conduits represented 

 by the irregular dikes and plugs. No doubt these all unite with the main 

 dike at no great distance from the present surface. The ramifications of 

 the eruptive channel must have caused the adjacent shale to lie highly and 

 uniformily heated, leading to a quite uniform rate of cooling for the magma. 



Description. — The Ivalston dike rock is similar to that from Vahnont, 

 but seems at first glance less distinctly porphyritic, because its augite 

 crystals are smaller, and the darker, fine-grained mass of the rock obscures 

 them, (hi closer examination, however, the structure is seen to be more 

 nearly the normal porphyritic than in the first case, for augite, olivine, 

 and a considerable number of plagioclase tablets are distinct phenocrysts 

 in an evidently crystalline but megascopically unresolvable groundmass. 

 The largest phenocrysts are tablets of labradorite 0.5 cm. across. 



By microscopical study it is found that the Ivalston dike rock has the 

 same mineralogical constitution as the Valmont dolerite, and the difference 

 between them lies chiefly in the development of the plagioclase. Here 

 there is a decided contrast between the phenocrysts of probable labradorite 

 and the smaller crystals of the groundmass. The latter are short and stout, 

 and nearly all have an irregular /.one of apparent orthoclase about them. 

 This is much more distinct than in the Valmont rock and its identification 

 as orthoclase seems unquestionable. There are many irregular grains of 

 orthoclase independent of the plagioclase. 



The augite phenocrysts are very irregular in form, and seem either to 

 have suffered resorption which destroyed their symmetry or they never 

 possessed sharp crystallographie boundaries. There are many very small 

 irregular grains, as in the preceding rock. 



Reddish-brown biotite shows a very strong tendency to form a ragged 



