As an undergraduate at Cambridge, I did the first-year NatSci Geology course which included a field trip to the Isle of Arran, led by the up-and-coming Simon Conway Morris. (I can date it to the last week of March 1987, because I remember hearing the news that Patrick Troughton had died.) It was great fun, clambering over rocks in the daytime, drinking with fellow students in the evening, but it failed to make me into a geologist. (Where I really failed was on palæontology. I cannot tell different types of fossil apart. I have the same problem with types of car.)
On one particular day, I was with a group that did a long coastal walk to Lochranza at the northern end of the island, including not only the spectacular footprints of Arthropleura, the largest land invertebrate of all time, but also Hutton's Unconformity. Here, we were told, Hutton had identified the difference in dip (the angle of the strata) between the Precambrian and Carboniferous rocks, and had realised that a very long time - more than 200 million years by modern reckoning - would have been needed for the Cambrian rocks to be laid down, thrust up on their sides, and then eroded down to the point that the Carboniferous sediments would start to settle on them. We take it for granted now, but the time periods involved are pretty mind-boggling.
We were slightly misled, of course. Hutton's trip to Arran wasn't as decisive for his thinking as a subsequent trip to Jedburgh in the Borders, where the real Hutton's Unconformity is located. Baxter's book also makes clear that Hutton's struggle to get acceptance for his ideas of the true timescale of the history of the earth was in part due to his own inability to express himself clearly, and his lack of time due to his own economic commitments as a failing farmer.
Baxter is very good on the social environment of Hutton's world - the 1745 conflict (when Edinburgh fell to the Jacobites with hardly a shot fired, and loyalist forces were routed at nearby Prestonpans, before the humiliations of Derby and Culloden), and then the slow reconstruction of Scotland as a polity, particularly via the Royal Society of Edinburgh, where Hutton was close to David Hume and Adam Smith. Baxter also observes of the sexual politics of the time - Hutton had at least one illegitimate child, who was provided for but sent to London. I feel he misses a bit the economic impact of Hutton's work - Wikipedia has more about his work on the Forth and Cyde canal than Baxter does.
The whole is as usual in Baxter's calm and lucid style, and if the point is to establish Hutton as a real human being grappling with the Enlightenment as it played out in Scotland, rather than as a heroic scientist deliberately intending to change humanity's view of the age of our planet, he has succeeded.