During a field survey of a desert environment, a geology stu…

During a field survey of a desert environment, a geology student collects a tan to reddish-brown rock that feels gritty to the touch. When rubbed, the sample leaves behind a fine sandy residue. The grains are cemented together but still easily visible, and the rock breaks apart along irregular surfaces rather than smooth layers. No fossil imprints are observed. The rock does not react with acid.

While mapping an area of ancient volcanic terrain, geology s…

While mapping an area of ancient volcanic terrain, geology students encounter a dark-colored rock that is fine-grained and very dense. The rock breaks with a conchoidal fracture and has a dull to slightly glassy luster. No visible crystals are apparent without magnification. No gas vesicles (small round holes) are found in the sample. The rock does not react with acid and shows no evidence of layering or foliation. Based on these observations, which type of rock has most likely been found?