I came across this in the file of ancient reprints. It's a translation of an article written by OT Jones in 1966 -- and it describes how the bluestones - Stonehenge connection was initially made by the geologists.
It all started on a grand day out in the summer (one presumes) of 1908, when Thomas, Cantrill, Strahan and Jones made a jolly train expedition to Rosebush railway station, walking from there to Crymych along the Mynydd Presely ridge before taking an evening train home from Crymych railway station on the "Cardi Bach." They obviously looked at a lot of rocks on the way. DC Evans was obviously their guide, and I have read elsewhere of his role in supplying material later on for HHT to look at.
Between that first trip and the serious work on the bluestones in 1918, ten years elapsed, and the First World War disrupted everything. Maybe it was the end of the war that provided the impetus.
Note that Jones, in his Welsh-language article, calls the stones "Cerrig Llwydion" -- not bluestones but GREY STONES.