Source: Rebecca Gowers. Plain Words (2014 ed). p. 222 Middle.
TROUBLE WITH VERBS
(1) ing endings
Words ending in ing are mostly verbal participles or gerunds, and, as we shall see, it is not always easy to say which is which. By way of introduction it will be enough to observe that when they are of the nature of participles they may be true verbs ('I was working') or adjectives ('a working agreement') or in rare cases prepositions ('concerning this question') or conjunctions ('supposing this happened'); if they are of the nature of gerunds they are always nouns ('I am pleased at his coming') — or rather a hybrid between a noun and a verb, for you may use the gerund with the construction either of a noun ('after the careful reading of these papers') or of a mixture between a verb and a noun ('after carefully reading these papers'). It is most confusing, but fortunately we are seldom called on to put a label on these words, and so I have preferred to give this section an indeterminate title.
'Carefully' is an adverb, and must qualify a verb. So how can the emboldened reading possibly be a noun, much less a mixture?