1 82 Tlie American Geologist. September, 1900 
subject of the next chapter. In the latter and in the normal 
diabase the texture is identical, both showing all gradations 
from a very coarse ophitic texture tending to become granitoid 
through medium grained ophytes and porphyrytes to the finest 
diabasic, and sometimes porphyritic basalts, without, however, 
usually containing a glassy residue. The intrusive character 
of the diabases was first distinctly announced by Lawson* 
whose views on this point are now generally accepted. The 
intrusive sheets are usually nearly flat and vary greatly in 
thickness, from a meter to one hundred meters or even more. 
They always present a striking subcolumnar structure at right 
angles to the plane of their extension, and form a prominent 
feature of the topography of northeastern Minnesota. These 
sheets, termed by Lawson "Logan sills" are especially com- 
mon in the region around Pigeon point where they are found 
interbedded with Keeweenawan as well as Animikie forma- 
tions. They are of the nature of horizontal dykes derived, ac- 
cording to N. H. Winchellt from the gabbro magma. Their 
composition is certainly not opposed to this view since both 
mineralogically and chemically they are practically identical 
with the gabbro. 
But diabases in northeastern Minnesota are not restricted 
in occurrence to these Logan sills. They occur also in numer- 
ous dykes and in masses of irregular shape. They occupy 
large areas as the country rock (see the map), and have been 
shown to belong to at least two different epochs. Still an- 
other category of diabases can be attributed to merely local 
variations of the ordinary gabbro, where the latter has cooled 
more rapidly than the main mass. In such cases the diabase 
is always very coarse and may show passages to true gabbros. 
(See plate IX. figure i.) To this category must be attributed 
the diabase studied here; it comes from near the northern 
border of the gabbro area, and it was perhaps the contact 
which caused slightly more rapid solidification. Neverthe- 
less the type presents no evidence of the slightest modification 
other than the change of texture ; in mineralogical and chemi- 
cal composition it is entirely comparable to the normal gab- 
bros. 
*A. C. Lawson: The laccolitic sills of the northwest coast of lake 
Superior. Geol. Nat. Hist. Surv. Minn. Bull. No. 8, 1893, p. 24. 
fN. H. Winchell: Geol. Nat. Hist. Surv. Minn. Final Rep., Vol. IV. 
