370 The American Geologist. jure, 1901. 
crustal disturl)ancc. such as the Ozark region of ^Missouri or 
the great mountain axes of the west : and so the rocks are witli- 
out the folds which are so essential to the accumulation of the 
fluent hydrocarbons. Besides the Trenton limestone there are 
petroleum-bearing rocks in other formations in Iowa, notably 
in the Carboniferous ; but so far as discovery has gone, some 
of the conditions on which accumulation in commercial quan- 
tities depends, are always absent. Usable quantities of gas have 
been found at a few places in Iowa in the drift. This gas has 
its origin in the buried forests ; beds of sand and gravel consti- 
tute the reservoir ; and overlying bowlder clay is the impervi- 
ous layer. Near Herndon and Letts are wells of this kinrl. 
The volume of gas is small ; its source is near the surface ; 
nothing would be gained, but much might be lost, by deeper 
borings. If either oil or gas is ever found in Iowa in usable 
quantities, outside the drift, it will be found either in or above 
the Trenton. There is no possibility of its occurring below that 
formation. Xow. remember that deep \\-ells which have pene- 
trated the whole thickness of the Trenton and gone hundreds 
of feet below it, are scattered all over Iowa. Every one of 
these wells, no matter for what purpose it was made, is, in ef- 
fect, a test hole for gas and oil ; and every one of them answers 
the question of the occurernce of these products in a way that 
might be inferred from what is known of the geological struc- 
ture — namely, in the negative. The state has been very thor- 
oughly explored beyond the deepest point at which there is the 
slightest hope of success, and a thousand other test holes would 
not make the situation any clearer or the results more decisive. 
There is always the very remote possibility that there may be a 
.small arch somewhere which has not been pierced b\' the drill, 
but the chances of its existence are so few. that if the object is 
simply to test for gas or oil. it would be an unjustifiable waste 
of money to search for it even if holes could be bored every- 
where down into the Trenton limestone at the rate of one dol- 
lar apiece. The geological structure of the state, in its broader 
features, is now thoroughly known. The records of the many 
" deep wells, so fully and accurately described b}- Norton in \'oi- 
ume \\ of the Iowa Reports, reveal that structure in scores of 
places down to the Algonkian : and from the base of the Al- 
gonkian to the earth's center, there is nothing but barren, igne- 
