Modern Igneous Rocks and the Primary Formations. 107 
slate, but on examination at the locality, it proved to be nothing 
but decomposed basalt. The chrysolite, which was very indis- 
tinctly seen in the basalt itself, had become stained with iron 
through partial decomposition, and was split into thin scales, and 
the whole deposit had received, in consequence, the foliated struc- 
ture of mica slate. ‘This foliation had taken place parallel with 
the top of the bed of basalt—a semicolumnar variety —and seemed 
to evince that the crystals of chrysolite, while forming, assumed 
parallel positions as above explained, with the face of most perfect 
cleavage horizontal. ‘The compact basalt could be chipped off 
with the hammer more easily at right angles with the columns 
than in other directions. ‘The chrysolite therefore, was in this case 
the mineral on which the cleavage depended. In instances of 
what are called concentric structure, the cause is the same. ‘The 
mineral upon which the concentric lamination depends, lies with 
its plane of most perfect cleavage, coincident with the plane of 
lamination. 
In slaty trachytes, the lamination may often be distinctly traced 
to the feldspar or crystals of hornblende or mica. Large crystals 
of glassy feldspar, often lie in the plane of lamination, beautifully 
illustrating these principles. ‘The hexagonal tables of mica have 
the same position, and when abundant, it produces the most slaty 
irachytes that are known. A fine illustration of the whole series 
of rocks from granite to mica slate, is presented by an extinct vol- 
cano in the Sacramento Plains, in Upper California. Much of the 
rock resembles granite or gneiss—although properly a trachytic 
porphyry. It consists of large crystals of glassy feldspar, dissem- 
inated thickly through a greyish base, which is speckled with 
small black crystals of hornblende and mica. In another part of 
this extinct voleano, the rock breaks into laminz a third of an 
inch thick, and contains tables of mica lying as usual in the plane 
of fracture, while the feldspar is in very small crystals, imbedded 
in a compact feldspathic base. 
We might cite examples from the volcanic regions of Europe, 
but what has been already said, appears sufficient to establish the 
fact that modern igneous rocks are laminated, and in general, 
more or less so according to the quantity and cleavability of the 
cleavable minerals they contain. Mica, the most perfectly foliated 
mineral, produces when abundant, and when not overruled by 
the other constituents, the most perfectly laminated rock. 
