求翻译：以往研究表明（吉林省地质局，1979），该旱玛珥由上至下的沉积层序如下：（1）草根层，厚度约20-30 cm，由植物根及未分解的植物残体组成；是什么意思？

Previous studies have shown (Jilin Province, Geological Bureau, 1979), the sedimentary sequence of the Maar from top to bottom as follows: (1) grass-roots layer thickness of about 20-30 cm by the plant roots and undecomposed plant residues ;

Previous studies have shown that (geological, Jilin Province, 1979), the ecological communities of the layer of sediment sequence is as follows: (1) grass-roots level, the thickness is approximately 20 - 30 cm, with plant roots and not break down vegetation debris;

Formerly studied indicated (Jilin Province geology bureau, 1979), this dry Masurium arc of contact of halo top-down sequence of sedimentation was as follows: (1) stock level, thickness approximately 20-30 cm, and the decomposition plant remains has not been composed by the plant root;

Previous studies have shown that (Jilin provincial Bureau of geology, 1979), dry Maar from top to bottom in the sedimentary sequence is as follows: (1) grass-roots level, the thickness of about 20-30 cm, by plant roots and are not composed of decomposing plant residues;

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