208 Wisconsin Academy of Sciences , Arts and Letters. 
Further light is thrown on this question by the fact that in its final 
advance the ice did not overrun this particular region. Hence whatever 
accumulation there was as a result of inter-glacial decomposition was 
preserved to be added to by the same disintegrating forces in post-glacial 
time. 
If one were inclined to jump at conclusions, the inference would be 
easy, that since rocks very similar to this, both in character and age, 
within the region covered by the ice in its last advance show almost no 
accumulation of decomposed material, while this rock, perhaps equally 
enduring, but lying just without the area of the last advance, is so 
deeply decomposed, therefore the time between the second and the last 
advance of the ice is much greater than that which has elapsed since its 
final retreat. 
This evidence would have more value as a time measure if there were 
other exposures of the same rock within the adjacent latest glaciated 
area. Unfortunately none occur. Of those that do occur from the 
granites near Big Stone Lake to the basic eruptives at Granite Falls no 
one of them, so far as can be learned, exhibits any appreciable accumu¬ 
lation of decomposed material. The same is true of this class of rocks 
in Wisconsin and northern Michigan and generally throughout the 
northern United States and Canada. 
So far as it goes, then, the testimony of this rock falls in with that 
which has been derived from comparisons of the till of the earlier and 
the later glacial epochs. These comparisons all go to show that inter¬ 
glacial time was vastly longer than post-glacial time. 
Petrograpliical Notes .-—The fresher rock is a very coarse-grained 
olivine diabase. The hand specimen shows lath-shaped twinned feld¬ 
spars over a quarter of an inch in length, sometimes with a dull green¬ 
ish hue as though partly changed to saussurite. Large columnar augite 
crystals, penetrated in all directions by feldspar, show in some cases 
cleavage faces over an inch in length. Spotting these cleavage surfaces 
are small yellow-green grains of olivine. Considerable black ore ma¬ 
terial and a little pyrite can also be made out under the lens. After the 
rock was pulverized the black ore material was strongly attracted to a 
magnet and when dissolved in hydrochloric acid in presence of tin gave 
no titanium reaction. 
The microscopicai study was made on two sections from one of the 
bowlders near the bed of the stream. In both the diabase structure is 
typically developed, the lath-shape fe Idspars penetrating the other con¬ 
stituents in all directions. No evidences of disturbances are visible, ex_ 
cept such as are explained by movements in the partially consolidated 
magma. There are present besides feldspar, augite, olivine, hornblende, 
biotite, ilmenite, apatite and clorite. 
The feldspar is poly synthetically twinned according to both the ordi¬ 
nary laws, and is only moderately altered. Determinations by Michel- 
