The area under consideration, shown on the key map, Fig. 1, embraces the northern portion of what I have called the evergreen Quadrangle, about 180 square miles, and the · southern portion of the Black Hawk Quadrangle, about 60 square miles. I have also covered in great detail, as far 'as the dikes are concerned, the region embraced in the so-called Idaho Springs and Central City special maps, an area of about 18 square miles.