英语翻译翻译：These traverses（横渡）are subjects of considerable anxie

英语翻译
翻译：These traverses（横渡）are subjects of considerable anxiety ,and frequently of delay to travelers.Being sometimes of considerable extent,varing from four and five —and in such immense seas as lake Superior —to fourteen miles.
frequently of 这是个什么结构啊?怎么副词后还能加of的啊?还有一句,前面说有河,然后说to save rounding which teavelers usually strike aross from point to point ,making what is called in voyageur parlance a taaverse.strike aross 为什么不用cross?为什么要用strike 好多不懂啊~我做卷子的快疯了

这里的of是与前面的subjects连在一起的,就是These traversesare subjects of considerable anxiety ,and frequently （subjects）of delay to travelers,意思是“横渡者十分焦虑,旅行时经常会延误”（上下文信息不足,可能翻译得不太准确）
至于to save rounding...那一句,strike across就是“穿过、通过”的意思吧,这里用cross或者across都是一样的.strike不仅有“打击”和“罢工”的意思,还有其他很多意思的（包括“穿过”、“到达”）,最好是找一本大点的字典看看.
另外,我觉得阅读理解不要拘泥于个别的句子,只要理解整体的意思就行了