exercised at the end of 40 years, or at the end of any 10-year period thereafter, up to 99 years from the commencement of operations. GEOLOGY AND TOPOGRAPHY. Monterrey lies in a plain at the foot of the Eastern Sierra Madre Mountains which constitute the eastern margin of the Mexican Cordilleran Plateau, and is surrounded by the magnificent mountains of that group, among the most notable of which are the beautiful Mitra and Silla Mountains. In the neighborhood of Monterrey these mountains attain heights of from 2,000 to 2,400 m., and are noted for their broken and jagged sky-lines. The leading geological characteristics of the district are the uplifted limestones of the older cretaceous age which form the main mass of the mountains. Primarily, the mountains are compressional folds which, in the Sierra Madre, near Monterrey, are close and vertically compressed.[2] The drainage areas of the Santa Catarina River, which flows through Monterrey, and of the Estanzuela and Silla Rivers, its tributaries, are of limestone and shale; originally the shales were above the limestone, but the convulsion which formed the Sierra Madre as an anticlinal fold, left the originally horizontal strata standing nearly upright, and subsequent erosion in the upper part of the anticline has exposed nearly vertical strata in many places. The limestone being hard and resisting erosion, there is generally, along the line of contact, an abrupt drop vertically on the face of the limestone to the shale below. In many places this abrupt drop is broken by a limestone talus, but the line of contact can generally be traced. Mining operations in these mountains have revealed the presence of large caves at a considerable elevation, many of which contain large reservoirs of water, delivered to them through numerous faults. The river valleys are formed of masses of limestone conglomerate and coarse gravels, re-cemented in many cases by the lime deposits of the flowing waters. One of the chief characteristics of the subsoil of Monterrey itself is a local rock called "sillar," which is a superficial deposit of carbonate of lime from the evaporated waters. In some places the "sillar" is largely mixed with a conglomerate called "tepetate," or "impure sillar." [2] _Transactions_, Am. Inst. Min. Engrs., Vol. XXXII (1902), pp. 163-178. [Illustration: PLATE II.--GENERAL PLAN OF THE WATER SUPPLY AND DRAINAGE WORKS FOR MONTERREY, N. L., MEXICO.]