Problems with an in-vent column collapse model for the emplacement of massive volcaniclastic kimberlite. A discussion of 'In-vent column collapse as an alternative model for massive volcaniclastic kimberlite emplacement: An example from the Fox kimberlite, Ekati Diamond Mine, NWT, Canada' by Porritt et al. [J. Volcanol. Geotherm. Res. 174, 90-102]