Coarseness of Igneous Rocks — Lane. 67 
If the contact zone is a broad one not cooling rapidly, 
but letting the plane of contact remain for some time in 
conditions just half way between those the country rock and 
the igneous rock had at the beginning (o) and Ca.^«v), the 
grain will continue to increase though not at a uniform rate, 
until it may even be a good deal coarser than farther from 
the margin. This belt of coarser grain not at, but parallel 
to, the margin will be pronounced only in rather exceptional 
conditions, when the contact zone is quite broad, and the 
temperature of consolidation only a little nearer the temper- 
ature of injection than that of the country rock. 
After a short time if the contact zone is a narrow one, 
a longer time if it is broad, the contact zone will be heated 
up so that the igneous rock will cool as though it were the 
outside of the contact zone which was its margin held at a 
fixed temperature, and if the contact zone is very narrow, 
as it generally seems to be in efifusive rocks, tlie grain in- 
creases once more uniformly. It will do this more especially 
if the temperature of consolidation is pretty close to that of 
the molten rock when it came to rest. This seems frequent- 
ly to be the case with effusive basic lavas which flowed on 
and on and stirred themselves up until the earlier crystals 
began to form while the lava was still in motion. Tlien it 
began to stiffen, and at this time and temperature it was on 
the verge of consolidation throughout. 
For instance the most common type of Keweenawan 
trap is an ophyte mainly composed of augite and labra- 
dorite with some iron oxides and olivine in chemical compo- 
sition an auvergnose close to a hessose, with about 47 per 
cent SiO^, 10 per cent CaO and 3 per cent alkalies. The 
type is the great ridge known as the "greenstone." Such 
flows continued to spread until the olivine and probably 
also the feldspar had begun to form. We find in them that 
the augite is the last formed mineral, and that its grains en- 
close the tablets of feldspar, but crowd ahead of them the 
other components so far as possible. These augite patclies 
attain a perceptible size as will be seen by plate W in four 
to ten feet from the margin. This corresponds to the first 
rale of increase. Thereafter the increase of size of patches 
is less rapid but fairly uniform and continuous, something 
