Furthermore, the crystalline character of rock 'A' and the increase in grain size from its marginal to central parts (indicative of faster cooling at the edges and slower cooling in the interior) support origin through cooling and crystallization of a melt.

The altered texture and mineralogy of the sedimentary layers adjacent to the intrusive can be attributed to elevated temperatures imposed by the melt.

But an alternative hypothesis might be that rock body 'A' originated through precipitation of minerals from a hot watery solution, such as heated groundwater, rather than through crystallization from a cooling rock melt. In theory, water injected under high pressure could exert lateral pressure to cause folding and slaty cleavage. Heat from the solution could allow the adjacent rocks to behave plastically. Chemical reactions between the solution and the surrounding rocks could produce the altered zones.

MELT	SOLUTION

The igneous origin of some rocks is indisputable: those seen to form from the solidification of erupted lava. It is judged 'indisputable' because it can be seen to happen. Can a connection be made between these rocks and others whose origin can not be observed? Examine the following diagrams and narrative to see if they supply an answer to that question.

Hypotheses: The dark blue layer is an igneous intrusive formed by molten rock similar to that which erupted as lava. The 'zones of unique character' are metamorphic, produced by the baking of the sedimentary rocks in contact with molten rock. 'Alteration Zones' seems an appropriate term for them.

Hypothesis: The geologist cannot see the feeder pipe and associated alteration zones (4), but assumes it must be there as there had to be a conduit to convey molten rock to the surface. That same conduit may have also fed molten rock to the horizontal intrusive.

Observation: The hill described above sits between two other hills. The sequence of rocks of which these hills are composed is the same as those that make up the first hill, except that two additional sedimentary layers sit on top of the lava flows layer.
Hypothesis: All three hills were once joined but have been separated by erosion.
Hypothesis: At some depth beneath the surface, there must be a magma chamber. It has probably cooled and solidified since there has been no volcanic activity for a long time. The length of time since volcanic activity ceased is judged to be long because a long time is probably required to erode the material that once joined the hills together.

Observation: The area is underlain by horizontal sedimentary rock layers. Over a large area, the sedimentary rocks have been invaded by a great irregular mass of coarsely crystalline rock whose mineralogy (olivine, pyroxene and calcium-rich feldspar) is similar to that of the previously described lava flow and dark blue horizontal intrusive rock. A similar rock mass of smaller size occurs nearby. Both rock masses are surrounded by alteration zones.
Hypothesis: These rock masses are part of a huge solidified magma chamber that, when it was molten, fed the lava flow and the horizontal intrusive rock body described above. These bodies are visible because in this region, more material has been removed by erosion.

In the preceding argument, the geologist started with rocks bodies whose origin is observed. Then, (s)he used identical or similar features found in other rock bodies to infer their origin. Follow the steps of the argument as shown in the diagram and narrative:

Igneous rock bodies are recognized by their shapes, structures, and relationships to other rock bodies in conjunction with textural, mineralogic and experimental evidence. None of these characteristics on their own are definitive. Crystalline texture, intrusive character, zones of alteration, the presence of a particular mineral or minerals, vesicular glass, or even a cratered cone may be open to alternative explanations. But as the number of these features found together rises, the probability of igneous origin increases rapidly to the point where the geologist may point and say, with justified assurance, "this rock is igneous."

Investigation then focuses on determining the precise conditions of magma formation, when and where they are likely to be achieved, details of the solidification of melts, and mechanisms of volcanic eruption.