Some special difficulties have stood in the way of making collections of the plants of the . These clays have come to be a most important element in the resources of the State, and they are the basis of a great industry. The clay beds have been opened at a great number of points, and as the different layers are the products of changing physical conditions, and probably mark the lapse of considerable intervals of time, it is not strange that many differences should be noticeable in the fossil plants of the various beds.
— JOHN STRONG NEWBERRY