Granite

Rounding off the story about granite started at reference 1 and continued inconclusively at reference 2.

The story was resumed by remembering about reference 3, an easy-going introductory text for undergraduates. The third edition of what appears to have been - possibly still is - a standard text. A large, well produced paperback from Freeman of San Francisco of some 600 pages. A book which I bought second hand years and years ago.

Chapter 4, 'Weathering: The decomposition of Rocks', contains the following: 'People in temperate regions are used to thinking of granite as the most permanent of rocks, but those in the humid tropics know that many granite boulders in soil can easily be kicked into a heap of mineral grains  ... The white to cream coloured clay is the mineral kaolinite (sometimes just called kaolin), used in pure form as raw material for pottery and china'.

Reading around this chunk, I now think it is a very likely candidate for the seed of the memory flagged up at reference 1. And I have been reminded that, often, a good text book, is a better way of learning about something than poking around on the Internet. A good text book offers a substantial, rounded story which you are unlikely to get from the Internet, a story which, in this case, starts at the beginning with the earth aa a sphere containing a lot of iron (35%), evolving to have a crust which contains a lot of oxygen (46%) and silicon (28%), a lot of that in the form of silicates, quartz and sand.

I have learned that granite is an intrusive rocks, pushing up into the crust from the depths, while basalt is an extrusive rock, coming from volcanoes and the like. I think that a lot of the differences between the two arise from the different circumstances of their cooling. Which intrusion might account for the inselbergs of references 2 and 4. Which appear to be largely confined to the drier parts of Africa, where erosion of the granite will be slower than it is just below the surface in the humid tropics. Unfortunately, the word does not appear in the index of the textbook. But they do get a mention at the helpful reference 5.

Gemini, however, is having a bad day, which he freely admits when I challenge him on his first response, snapped above. They don't seem to be able to cure him of his tendency to agree with whatever you say.

PS: I forgot about China Clay, one of the products of rotting granite and for a while a very important industry in Cornwall, even making it to the FTSE 100. See references 6, 7 and 8. Then, on some whim, I went to look for Charlestown of reference 8, once West Polmear. Neither Ordnance Survey nor gmaps seem to have heard of this Charlestown, but Wikipedia offered reference 9.

I eventually run the place down using the Scottish maps. It turns our that East and West Polmear are some distance apart, roughly the top right and top left corners of St Austell Bay. The top right corner, Par Sands, is a place I believe I visited as a child when on holiday in Cornwall - although I have no memory at all of the China Clay mountains dotted about the area. I failed in the present search for Charlestown by concentrating on Par Sands, despite it not looking like a great place for a harbour (from which to ship out China Clay).

Polmear plain and simple is all that is left now of the two Polmears, above the top right corner of Par Sands.

Whereas the harbour is all present and correct in Ordnance Survey when you look in the right place. Left in the snap above. With the Eden Project, itself a disused China Clay pit, upper centre. Lots of other China Clay heritage scattered about.

And gmaps does find the almshouses mentioned at reference 9, behind the van in the snap above. I suppose the trouble was that Charlestown is a customary name, rather than a name recognised by central or local government in a borough or parish. Perhaps the Charles Rashleigh for whom the place is named did not put enough money into the right pockets, back in the day.

References

Reference 1: https://psmv5.blogspot.com/2024/12/festive-lacey.html.

Reference 2: https://psmv5.blogspot.com/2024/12/festive-pudding.html.

Reference 3: Earth - Frank Press, Raymond Silver - 1974, 1982. 

Reference 4: https://en.wikipedia.org/wiki/Inselberg.

Reference 5: Geology and Landforms: Dartmoor Factsheet - Dartmoor National Park - 2005. https://www.dartmoor.gov.uk/__data/assets/pdf_file/0019/72109/lab-geology.pdf. Kopjes, aka inselbergs, get a mention here.

Reference 6: https://en.wikipedia.org/wiki/Kaolinite.

Reference 7: https://cornishstory.com/2021/01/02/the-china-clay-industry/.

Reference 8: https://www.cornwalls.co.uk/history/industrial/china_clay.htm.

Reference 9: https://en.wikipedia.org/wiki/Polmear,_Cornwall.