Petrified wood consists of a wide variety of mineral including silica, silicates, carbonates, sulfates, sulfides, oxides, and phosphates. They all can per-mineralize wood to form petrified wood. However, petrified wood most commonly consists of silica in the form of either opal or chert. Silicified wood is usually found within one of two types of strata. First, it occurs within accumulations of volcanic ash, tuff, ad breccia, e.g. the petrified forests of Yellowstone National Park. Second, silicified wood also occurs within sands, silts, and muds deposited by rivers and streams that have hardened to sandstones, siltstones, and shale. The silicified wood found in Petrified Forest National Park in Arizona and the Miocene strata of Louisiana and Texas occur within such strata. It forms in these deposits, because of the presence of dissolved silica within the groundwater. The silica is derived from the dissolution of the volcanic material by the groundwater within the sediments. Within 10 and 40 million years, the opal of the silicified wood further dehydrates and crystallizes into microcrystalline quartz (chert). Factors such as temperature and pressure may speed or slow the process, but eventually the opal of the silicified wood becomes chert. During the change from opal to chert in silicified wood, the relict woody texture may either be retained or lost.In southwest Texas and into Louisiana, three types of silicified wood can be recognized. First, the “nondescript silicified wood” is one that possesses a recognizable woody structure. It is not identifiable without oriented thin sections, specialized references, and comparative material. Therefore, this type is best described just as “silicified wood”. The second type, “Palm Wood”, is a group of fossil woods that contain prominent rod-like structures within the regular grain of the silicified wood. Depending upon the angle at which they are cut by fracture, these rod-like structures show up as spots, tapering rods, or continuous lines. The third type is “massive silicified wood” in which the silicification of the wood, or subsequent transformation of silica gel to opal or chert, has obliterated any trace of the grain of the former wood. Because of its variable, massive nature and heterogeneous trace element composition, many investigators have often failed to recognize the nature of this material.1.What does the passage mainly discuss?2.Which of the following is commonly a part of petrified wood?3.The underlined word “strata” in Paragraph 1 is closet in meaning to （ ）.4.It can be inferred from the passage that which of the following is true of Yellowstone National Park?5.The underlined word “such” in Paragraph 1 refers to （ ）.6.The author mentions groundwater (the underlined word) in Paragraph 1 in order to（ ）.7.According to the passage, the opal of the silicified wood became chert because （ ）.8.In paragraph 2, the author discuss three types of silicified wood because （ ）.9.The underlined word “prominent” in Paragraph 2 is closet in meaning to （ ）.10.Which of the following is a description, rather than one of the three type of silicified wood?11.What is the reason why many investigators have often failed to recognize the third type of silicified wood?

A.the nature and types of petrified wood B.the formation and location of petrified wood C.the composition and process of petrified wood D.the structure and transformation of petrified wood
问题2：
A.tuff B.chert C.strata D.silts
问题3：
A.locations B.structures C.textures D.levels of earth
问题4：
A.it was once a volcano B.it once had a wide variety of minerals C.it had no rivers or streams D.it had no groundwater
问题5：
A.sands, silts, and muds B.sandstones, siltstones, and shale C.silicified wood D.Arizona, Louisiana, and Texas
问题6：
A.show that not only temperature and pressure are important in forming petrified wood B.explain the beginning of the process that leads to sandstone, siltstones, and shale C.explain how silica is formed D.mention an important part of sediment
问题7：
A.of the influence of time B.of the influence o