"The Franklin Mountains extend from just north of downtown El Paso in El Paso County into southern Doña Ana County, New Mexico; their center is at 32°01’ north latitude, 106°32’ west longitude. They are roughly three miles wide by twenty-three miles long and rise to an elevation of 7,192 feet above sea level at North Franklin Mountain. The mountains divide the city of El Paso and have influenced its shape and growth. This range comprises the bulk of the second largest state park in Texas and what is said to be the largest urban park in the US, Franklin Mountains State Park.
The Franklin Mountains are composed primarily of carbonate rocks of the 1.25 Ga Castner Marble preserved as a roof pendant within a large granitic intrusions. Although metamorphism has destroyed the primary chemistry of Castner carbonates, depositional structures remain well-preserved. Geologists refer to them as tilted-block fault mountains. The Franklins were formed during the Laramide mountain-building period in late Cretaceous time, 60 million to 70 million years ago.(…)”
Crystal twinning patterns in a leucite crystal from volcanic rock, observed in polarized light by Dr. Michael M. Raith of the Steinmann Institut, University of Bonn, Bonn, Germany. (Dr. Michael M. Raith)