The origin of most rocks cannot be observed: they form at depth within the earth or in the past before people existed. To explain their origin, geologists draw analogies with what they can observe. They weave a web of circumstantial evidence, depending upon a confluence of lines of argument to finally draw conclusions that over time stand up to scrutiny with sufficient resilience to be considered 'highly probable'.
Consider the origin of an intrusive body. Many alternative explanations have to tested before one is finally deemed acceptable. Criteria dealing with compositional, textural, and structural details as well as relationships with other rock bodies in the field have to be met. It seems a lengthy and arduous process, with results that fall short of 'proof'. Faced with the task of understanding all the myriad features, with all their seemingly infinite variations that collectively make up the earth, how can the geologist and the science of geology make any progress?
Geologists, like all other scientists, and like professionals in many other fields, make use of what has been called 'a black box'. Each time a local problem is resolved, such as the origin of a particular intrusive body, it adds to a body of settled disputes. When a sufficient number of similar cases, after exhaustive investigation, have been resolved in the same fashion, when in each case the same conclusion has been drawn, the issue is no longer considered 'problematic'. The answer is considered obvious, the matter settled, and the explanation comes to be considered a 'fact'. The issue becomes a 'non-issue' and, so to speak, is put into a 'black box', out of sight, out of mind. Attention is focused elsewhere.
Answers to the problem of general origin of rock bodies, whether they are igneous, sedimentary or metamorphic, after an epoch of wonderfully passionate disputes, became largely settled by the middle of the 19th century. Since that time, during their training, geologists absorb the fruits of those disputes without having to engage in the arguments that led to their resolution. For the modern geologist, a cross-cutting body made of a crystalline intergrowth of quartz, feldspar and mica is automatically judged to be of igneous origin. A clastic mixture of quartz, feldspar and mica fragments is sedimentary. A rock consisting of quartz, feldspar and micas, with the micas segregated into parallel aggregates is judged to be of metamorphic origin, even if its field relationships are unknown. Such decisions become quick and instinctive, preludes to more ambitious inquiry.
Are such judgments ever wrongly made? Very occasionally. They come to light when later decisions based on those first judgments conflict with one another. Then the first judgments are revisited, criteria revised, and the science advances.
When a cross-cutting body is called an 'intrusive', regardless of whether the material is igneous or not, another important judgment has been made: the cross-cutting body formed after the host material was already in existence. First the host was formed, then the cross-cutting event took place. Time and sequence have been introduced into the narrative. Geologic history ('earth history' if you will), is being postulated. If the cross-cutting body is intrusive, one version of history emerges. If the cross-cutting body is not intrusive, another version of history emerges.
In scenario 6, the cross-cutting body is intrusive, and therefore younger than the surrounding rocks. In scenario 7, the cross-cutting body is not intrusive, and is older than the surrounding rocks.
Correctly interpreting the processes involved is vital to the emergence of a 'correct' history. (In this case, determining the nature of the zone rimming the cross-cutting body will be critical. Is it a lithified soil layer or is it metamorphic, a 'baked zone?) Conversely, if the timing of events could be independently determined, then history could help in the correct judgment of process.
Part 2 of 'The Fourth Dimension' deals with the meaning and determination of 'time' and how it assists in the construction of the history of the earth.