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 nineteenth century and then by several local geological journals in the twentieth century. As a logical consequence of this development, separate journals have now appeared aimed mainly towards either professional or amateur readership.

　　【补充】

　　first by ... and then by ....

　　为非限制性定语从句which is省略

　　=(which is) first by national journals in the nineteenth century and then by several local geological journals in the twentieth century.

　　which指代前方先行词the widespread introduction of refereeing

　　例如：

　　Now researchers suspect that dreams are part of the mind’s emotional thermostat, regulating moods while the brain is “off-line.”

　　= (which is)regulating moods while the brain is “off-line.”

　　which 指代先行词the mind’s emotional thermostat

　　参考译文

　　其总的结果是，业余人员更难在专业地质刊物上发表论文：加强广泛引入评审制度的结果。该制度开始是在19世纪为期刊所采用，后来在20世纪为几个当地地质期刊所采用。这样发展的逻辑结果是出现了针对专业读者和业余读者的不同杂志。