  A second peak in extreme ultraviolet sometimes appears during the gradual
phase of solar flares, which is known as EUV late phase (ELP). Stereotypically
ELP is associated with two separated sets of flaring loops with distinct sizes,
and it has been debated whether ELP is caused by additional heating or extended
plasma cooling in the longer loop system. Here we carry out a survey of 55
M-and-above GOES-class flares with ELP during 2010--2014. Based on the
flare-ribbon morphology, these flares are categorized as circular-ribbon (19
events), two-ribbon (23 events), and complex-ribbon (13 events) flares. Among
them, 22 events (40%) are associated with coronal mass ejections, while the
rest are confined. An extreme ELP, with the late-phase peak exceeding the
main-phase peak, is found in 48% of two-ribbon flares, 37% of circular-ribbon
flares, and 31% of complex-ribbon flares, suggesting that additional heating is
more likely present during ELP in two-ribbon than in circular-ribbon flares.
Overall, cooling may be the dominant factor causing the delay of the ELP peak
relative to the main-phase peak, because the loop system responsible for the
ELP emission is generally larger than, and well separated from, that
responsible for the main-phase emission. All but one of the circular-ribbon
flares can be well explained by a composite "dome-plate" quasi-separatrix layer
(QSL). Only half of these show a magnetic null point, with its fan and spine
embedded in the dome and plate, respectively. The dome-plate QSL, therefore, is
a general and robust structure characterizing circular-ribbon flares.
