The overall result has been to make entrance to professional geological journals harder for amateurs, a result that has been reinforced by the widespread introduction of refereeing, first by national journals in the 19th century and then by several local geological journals in the 20th century.

【翻译】总体结果是，业余爱好者欲在专业地质学期刊上发表论文就比较困难，而被广泛采用的论文评审制度又进一步强化了这一结果，这种制度首先由19世纪的国家级刊物采用，然后又在20世纪被几家地方级地质学刊物采用。

从结构看，first by national journals in the 19th century and then by several local geological journals in the 20th century，是 introduction of refereeing（论文评审制度的采用）的定语。由于定语太长，加之有 fist 和 then 排列，之前加了逗号停顿。

结构为：the introduction of sth. by sb. 某人对某物的引入采用。