If you want to find fossils, what kind of rock (igneous,sedimentary, metamorphic) should you look for? why?

Hoover Dam (Colorado)- Type of sedimentary rock that surrounds it.?

• I want to find out imparticular what Geographical Period the types of rock surrounding the area of the dam is. -If so, what kind of fossils (and their Period of age) would likely to be found here. -Or maybe during construction of the dam, were fossils very common?? Thanks!

• Answer:

  From a website I found: "The El Dorado Mountains primarily are Precambrian metamorphic rocks with intrusions that are about 1.4 billion years old. These old rocks are overlain by colorful (red, white, and brown) Tertiary volcanic tuffs, ash, pyroclastic flows, and basalt flows, mostly of Oligocene and Miocene age (40-20 million years old). The El Dorado Mountains were uplifted during the Miocene Basin and Range Uplift, about 15 million years ago. This is volcanic country, and Black Canyon gets it name from the black, volcanic rocks in the area." http://www.birdandhike.com/Wilderness/BlackCyn/Black.htm The area is comprised more of volcanic rock than sedimentary rock. Fortification Hill, which looms in the background of Hoover Dam, is an extinct volcano. Several visible layers in the mountain detail the many eruptions that occurred in the region over thousands of years. Because of its volcanic nature, I don't think any fossils were found during the construction of the dam which began in 1931. I was unable to find anything about fossils.