In the early 19th century, mineralogy and petrology were closely aligned with the study of formations, and the origin of crystalline rocks was the subject of debate in terms of larger geological theoretical frameworks. Two major developments revitalized petrography and petrology: the petrographic microscope (1850s-70s), and the rise of experimental petrology at the beginning of the 20th century. The two major readings provide a contemporary (1901) review of petrology, and a more recent (1996) overview.

Harris sect. 24 

Section 24 summarizes developments into the early 20th century.

Teall (1901) 

Jethro Teall was the leading British petrographer of his day, and was elected President of the Geological Society of London (this essay was his presidential address). He overviews his discipline and gives us a view of what problems and ideas he viewed as most significant. He also makes some very astute suggestions for future work. Please do not get bogged down in all the details of the various interpretations – you should skim a few parts.  Instead note what the controversies that faced petrologists, the methods of study, the types of proposed solutions, and the type of information available.

Here are a few notes on things that stuck me. The paper is divided into four parts:

1. Introduction (p. lxii-lxvi): Some interesting comments here – the idea that theories are useful when they coordinate facts; the advantage of multiple hypotheses; classifications that don’t related to genetic groups are unsatisfactory; the recognition that both Hutton and Werner made contributions and deserve recognition. (The contrast with Geikie is notable!)
2. Progress during the first half of the 19th century (p. lxvi-lxx): This section largely deals with the controversy about the origin of granite. As Werner’s aqueous precipitation model gave way, the question remained about the nature of the solutions involved. One interesting outcome (very end of section) is that this led to the question of the order of mineral consolidation.
3. Consolidation of Igneous Magma (lxx-lxxvii): Moving forward in time, the petrographic microscope allowed fine-grained igneous rocks to be examined. This led to suggestions regarding the order of mineral consolidation and questions about differentation. (Some of this gets a bit detailed – bottom p. 72-top 77 – and is best skimmed. There are a couple of calls for experimental work (p. lxxii, lxxvii) that are right on target – recall that the Geophysical Lab started in 1905 and Bowen began publishing his findings in 1915; Gibbs’ key chemical thermodynamics work was published in 1874–78.
4. Origin of [Mineral] Species (p. lxxviii-lxxxvi): The questions are really why are there so many different types of igneous rocks (versus variations in magmas), and why do compositions vary so much, even in one eruptive sequence. Teall gives a nice tour of some of the major ideas regarding differentation. Note the comments on classification (bottom of p. lxxx – the CIPW classification was only two years away when this was published.

Finally, I must note the reasonableness of this paper’s tone, and admire the honesty of the last sentence.

Oldroyd (1996) ch. 9 

Oldroyd’s book is a relatively recent overview of the history of geology. This chapter looks at minerals and rocks from pre-Wernerian studies through the mid-1950s (and later). Here are some comments to guide you through the main topics.

• You can skim or skip the opening pages (p. 192-200) which review ideas on the origins of rocks that will be familar to you, but which (rather maddenly) do not acknowledge the difference between classification (usually either based on chemical composition or placement in a Wernerian scheme) and identification (using Werner’s external characteristics, blowpipe tests, etc.), and also spends time using a botanical analogy (you may recall that classifications using simlar approaches were dismissed by geologists in the mid-18th century). He also traces some of the geological arguments back to the Werner-Hutton (aka Neptunist-Vulcanist) argument – implying that it persisted into the early 20th century – far longer than I would suggest.
• The bulk of the chapter deal with some major issues that are of interest to us in terms of developments from the mid-1800s onward:
  • The process of metamorphism and the formation of foliation (p. 200-207): This starts with Lyell’s recognition of metamorphism (at last). The discussion of Hunt (and the odd Eozoon story) divides the ideas before and after the petrographic microscope (feel free to skip Hunt p. 203-205 – he gets more prominence than usual in this telling).  Some of this section provides some insight as to why the petrographic microscope was so important.
  • Origin of granite (p. 207-212): Perennial favorite: magma, migmatite (“mixture” – essentially alteration by aqueous solution or very dilute magmas), or metamorphism (or all three?)
  • Differentation of magmas (p. 212-216): Another of Teall’s major topics from a different perspective.
  • And another Teall issue: tthe issue of space for emplacement of magmas (p. 216).
  • Impact of the Geophysical Lab and Bowen (p. 216-221): This section makes the importance of experimental petrology clear (I hope).
  • Ending with a revisit to the origin of granite (p. 221-223). Some arguments just never really end.

To dos

• Major topics
  • As you work your way through all this information and varied presentations, there are really three major topics that you need to address:
    • Metamorphic processes
    • The origin(s) of Granite
    • Differentation of magma and generation of different igneous rock types
  • For each of these, try to outline the major ideas (scattered through Teall and Oldroyd). How did they supposedly work? What kind of field data was being explained? I strongly suggest that you note the order of publication so you can see influences.
• What was the significance of the major developments in the late 19th to early 20th centuries?
  • Petrologic microscope (Sorby, 1858, was the real start for petrology)
  • Geochemical thermodynamics (Gibbs 1870s)
  • Experimental Petrology/Geophysical Lab (founded 1905)

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