苗栗縣境內汶水地區之第三紀沉積岩層中常有黑色發亮物質存在於岩層內。此種黑色發亮物質常為深黑色，表面光亮且平滑，具玻璃光澤。此種黑色發亮物質常有兩種產狀，一為與岩層層面平行者，另一種為黑色發亮物質存在於破裂面或生痕化石周圍。本研究主要之目的就是想要 了解此黑色發亮物質為何。探討其是否為假玄武玻璃或者為其他物質。 研究結果發現，黑色發亮物質出露的地層以第三紀中新世出磺坑層及北寮砂岩為主。含有此種黑色發亮物質的薄層，通常為砂、頁岩所組成的薄互層，其厚度常不超過1 公分，而黑色發亮物質形成的薄層，其厚度更是小於1 公釐。黑色發亮物質主要以碳質物、菱鐵礦、綠泥石、伊萊石及黃鐵礦為主，次要礦物為石英及長石。而本研究沒有足夠證據顯示該黑色發亮物質為假玄武玻璃。因此初步推論，斷層沿黑色發亮物質薄層滑動時，碾磨層面上的礦物，少數的礦物相成為非晶質物質。此種超磨作用使得黑色發亮物質薄層具有較高的可朔性，可沿著圍岩裂隙侵入圍岩間，形成本區黑色薄層的兩種產狀，平行層面者及侵入圍岩者。 由鏡煤素反射率值推估黑色層上的碳質物記錄下的最高溫度為379℃，推測該溫度為斷層作用時在黑色發亮薄層產生的摩擦熱。造成該黑色薄層的層面滑移作用可能不只一次。黑色薄層之圍岩溫度約150℃，經由地溫梯度及抬升速率換算，黑色層的可能形成時間至少在2∼0.5 個百萬年前(上新世∼更新世間)。推測附近的斷層(可能為司馬限斷層)活動時，帶動本研究區的岩層發生層面滑移作用，其活動時間可能與此年代相近。 There were usually had black shining material in the Tertiary sediments. Black shining material usually had smooth raw surface and displayed glassy luster. Two types of black shining material were found in this study: one present and parallel to bedding planes; the other was along fractures of the host rocks. The major object of this study was to know what the black shining material was, were it pseudotachylite or not? The results showed that black shining material were present in Chuhuangkeng Formation and Peiliao sandstone, Western Foothills Belt. Black shining material consisted of carbon material, siderite, chlorite, illite, pyrite and small part of quartz and feldspar. There were no enough evidence to prove that the black shining material were pseudotachylite. Fault crushed mineral which on black shining material becomes amorphous material during faulting. Then black shining material appears higher plastic to inject into fractures of the host rock. Therefore, we got two types of black shining material that were parallel to bedding planes and inject into fractures. Black shining material recorded the highest temperature of 379℃, this temperature maybe result from fault friction heating. Vitrinite reflectance value inferred that black shining material maybe existed at least 2~0.5million years ago. When fault motion (maybe Sumahsien fault) result in the strata occurred bedding-slip faulting.