68 The American Geologist. February. 1905 
like one millimeter in breadth of patch, diameter of augite 
grain, for every sixteen feet distance from the margin. 
Some valuable application can be made of these facts. 
(i) If a sample appears to be unusually line grained for 
its distance from the margin of the flow it is often found 
that it is more feldspathic than the normal ophytes (andose, 
i. e. "ashbed diabase, i. e. porphyritic melaphyre, or some- 
thing still more felsitic.) 
(2) The direction in which the mottles (grain) are get- 
ting finer is probably that of the nearest amygdaloid,* and 
the coarseness of the mottles enables one to infer about how 
far ofif (or ahead in diamond drilling) it may be expected if 
slips do not intervene. 
(3) As Dr. Hubbard suggests, if a flow appears by 
chemical or petrographic analysis to be a normal ophyte 
but when cut by a drill hole the grain does not increase at 
the normal rate it may be possible to infer the dip. In a 
vertical shaft or drill hole, the apparent rate would be the 
real rate of 16 feet per millimeter divided by the cosine of 
the dip. But the difficulty of determining the rate of increase 
of grain accurately, the liability of drill holes to veer, and 
the possibility of exceptional temperatures in the flow are all 
factors that should lead to extreme caution in the applica- 
tion of this method. The dip thus obtained should be taken 
as a suggestion. I have however used it with better suc- 
cess than I expected. 
(4) Sudden variations in grain may serve to call atten- 
tion to faults and slips. 
(5) Intrusive dikes behave quite dififerently. 
(B.) If the conditions of consolidation are nearer those 
of the country rock than those the rock had at the time of 
intrusion and the beginning of cooling, it will be coarser at 
the margin than at a little distance therefrom. If the con- 
tact zone is very narrow the zone of decrease of grain will 
be very small, and from thence in the grain will be as though 
the consolidation was from the outer margin of the contact 
zone kept at a fixed temperature. But if the contact zone 
is broad there will be a gradual decrease of grain to the zone 
of uniform grain of which we shall presently speak, the extra 
* The porous bubbly top of a flow. 
