From James Hastings Nichols, Corporate Worship in the Reformed Tradition, p. 102-103.

“Lecturing” was a running exposition of Scripture and was especially popular through the system of Puritan [i.e. not Presbyterian] “lectureships,” of endowed preaching posts outside the regular benefices.  The Directory permitted lecturing, but specified that Scripture was to be expounded, it should wait till the end of the chapter.  What was merely permissive soon became general practice.  The minister added an expository “lecture” to his reading of Scripture in addition to the sermon.  The Scots Assembly had to set the hour for morning worship half an hour earlier to accomodate the additional time added by the “lecturing.”