Volcanic Ash in the Clay Rocks from Upper Shaximiao Formation of Middle Jurassic, Northeast of Sichuan Basin: Evidence from Petrology, Mineralogy and Geochemistry是什么意思 Volcanic Ash in the Clay Rocks from Upper Shaximiao Formation of Middle Jurassic, Northeast of Sichuan Basin: Evidence from Petrology, Mineralogy and Geochemistry怎么读