I don’t think Hutton ever visited Schiehallion. I imagine now all his ‘travels’ occurring cerebrally; the young man strutting his stuff in his Newcastle school rooms, teaching and calculating, and the mature version based in Greenwich inventing methods of computing.
It’s a two dimensional vision, a paper version, yet he possessed volume; his maths was of volume, flow and trajectories. He may not have climbed mountains but he possessed the facility to imagine their internal structure. Perhaps from visiting the coal mine his father worked or he may have walked the Northumbrian coast where still excellent displays of strata, fault and basalt are seen to meet sea. He is a visual man and I like that, he imagines the interior of Schiehallion:
“…divided by vertical planes, forming an imaginary group of vertical columns, something like a set of basaltine pillars, or like the cells in a piece of honey-comb”.
When I think of lines in a landscape, I see sheep paths trailing round moor, strata sandwiched rock faces, or – of course – contour lines, roads and watercourses marked on maps: horizontality. 
For Hutton’s calculations of volume he needed first to ascertain ‘spot heights’, but he wasn’t seeing spots; each point was conceived as a column travelling down through rock.
He imagines an experiment: “procuring holes to be bored… for by such operations it is known what kind of strata the borer is passed through” … Solid stuff.
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