Dartmoor granite is to be more precise an adamellite (or a quartz monzonite (across the pond)). This means it has equal proportions of orthoclase and plagioclase with quartz constuting around 30%. Generally observers have suggested an igneous origin, which Ussher in 1888 also proposed was laccolothic in form. Hunt in 1894 postulated a metasomatic origin but he has virtually stood alone in this suggestion. The mass of Dartmoor was examined in detail by Brammal & Harwood in the 1920's and early 1930's. A coarse and a fine grained variety of granite had been known from earlier works. Brammal and Harwood divided the coarse variety into two further sub-divisions;

1) Characterised by abundant large feldspar phenocrysts (giant granite),
2) With only a few large feldspar phenocrysts (blue granite).

They also observed that the tors were formed of giant granite and the lower ground was underlain by blue granite, an observation that has since been proved incorrect.

At Hay Tor (SX 758771) a body of fine blue granite appears to intrude the giant granite of the tor itself, it's base is not exposed. They reasoned that this material represented the chilled upper margin of the blue granite which crops out on the ground to the west. They also suggested that the dark inclusions and basic segregations could be relics of an ealier dioritic phase broken up and incorporated by the magmas responsible for the giant granite phase and then for the blue granite phase. Fine grained granitic rocks form only a very small proportion of the granitic mass, occurring mainly as dyke-like and sill-like bodies. These were assigned to a fourth phase of activity followed eventually by quartz-tourmaline veining, greisening and local metalliferous mineralisation.
Modifications of the opinions expressed by Brammal & Harwood has been  necessary but their contribution has been of immense importance. They also believed a laccolithic form and igneous origin for the Dartmoor granite.

The Lands End granite between Porth Naven (SW355308) and Gribba Point (SW355302) shows a complete gradation between 'blue' and 'giant' granite. This was noted by Hawkes & Dangerfield in 1978, they also found that the blue and giant granites make up 90% of Dartmoor's exposed pluton and that the blue granite is not confined to the lower ground, as it actually makes up the highest parts north western parts of the moor. Mapping has shown the blue and giant granites to be variants of a single intrusive phase. Hawkes & Dangerfield suggested that three textural materials can be recognised in the Variscan granites of South West England;

1)  Coarse granite
2)  Medium grained lithium mica granite
3)  Fine grained granite

The coarse granite is best defined as that with a mean groundmass crystal size of about 2-3 mm. In the case of Dartmoor the coarse granite is characterised for the most part by large white feldspar crystals between 10 and 170 mm in length, making up about 30% of the rock's volume. As the percentage of megacrysts increases so does their mean length. They proposed 2 coarse variants both of which are very similar. Feldspars make up 65% of the coarse granite with quartz and 5-7% biotite. Plagioclase compositions vary in the Albite - Oligoclase range. The pottasium feldspar has developed by solid state replacement of the plagioclase in such a way that the host crystals may show any stage between minor or complete alteration. Commonly replacement has proceeded beyond the crystal boundaries  and merges with other crystals resulting in the expulsion of the matrix. Because of the secondary replacive nature of the potasium feldspar, amounts in specimen may vary between 30 and 40%. Plagioclase constitutes 20-30% of the rock by volume.

The shapes of the majority of the tor outcrops are closely controlled by major joint planes which fall into 2 obvious categories, those inclined at high angles and those that are sub-horizontal, usually termed floor joints. The origins of these two categories are probably different, a third group is less well developed and inclined broadly at angles between 20 and 80 degrees, these may also have a third cause. All the joints do though show one common feature, when viewed in quarry exposures or in the higher part of the tors, they appear in greater numbers per unit of rock volume towards the top.
Jointing is commonly regarded as a strain phenomenon produced in response to stored stresses, thus the increase in jointing upwards in the granite due to the removal of the overlying rock by erosion. Three types of joint can be seen in the granite mass. Floor joints are just about horizontal in ridge exposures and hilltops but are seen to incline up to 25 degrees towards valleys or depressions. These joints generally mirror the topographic form which itself may indicate the original shape of the pluton's roof.
The highest angle joints are seen with the same general directions in each of the granite bodies of South West England. They all appear to relate to regional stress fields associated with prolonged opening of the North Atlantic Ocean basin.
A third group of joints are not so well documented, however it seems that quartz porphyry dykes and metalliferous lodes occupy planes which may equate with many of the higher angle joints of this type. An explanation may lie in the fact that a high proportion of the Cornubian crustal block is according to geophysical evidence (Bott et al, 1970), composed of relatively light granitic materials. This must mean that there was considerable isostatic imbalance with respect to adjacent denser crustal blocks during late Carboniferous times. Attempts to achieve equilibrium may have resulted in vertical compression within the Cornubian block, the resulting stress fields causing the jointing of this third type.

The granites of South West England occupy a belt of country having a caledonide trend, but they are individually associated with Amorican / Variscan axes of uplift and were injected into the crust near a line of weakness bordering the ocean (Dewey, 1925). Direct dating is a problem with a large spread of data. Potasium / Argon dating puts emplacement at 303 Ma-265 Ma (Miller & Mohr, 1964), but using recent constant changes this comes out between 309 Ma and 271 Ma. However, fine grained granite pebbles can be found in red breccias of Stephanian age, meaning the granites were emplaced before the formation of these Stephanian Breccias.

A palingenetic origin is most favoured and geophysical evidence supports this view with a large mass approximating to granitic composition seen underneath the south west penninsula extending westwards. Further evidence comes from the high Oxygen 18/16 ratios found in granite samples at outcrop. A fair amount of caution must be exercised here though. The Ternary Minimum Theory states that given steady pressure conditions and a deceasing temperature, a granitic magma would differentiate to produce a residual liquid composition that upon crystallisation yeilds quartz, potasium feldspar and sodic plagioclase in roughly equal proportions. This is not the case with Dartmoor and the Orthoclase appears to be the result of later replacement. The Dartmoor granite would require isometric or adiabatic conditions requiring a relatively rapid ascent from deep crustal regions, and may possibly have had to find its way through the mass of granitic material believed to exist at depth.
Bott et al (1958) using gravity data in Dartmoor's case suggested magma rising in the south and spreading north in a laccolithic tongue several kilometres thick. However the form of the cornish granites suggest it is most likely that the magma rose through relatively resistant Devonian rocks, spreading out upon reaching the Carboniferous - Devonian interface to the north and to a lesser extent to the south. It is generally agreed that no exposures are at any depth within the pluton, there are plenty of xenoliths and partly consumed shale and contamination such as at Birch Tor. It is also thought that much of the giant granite of the tors resulted from contamination and an increased availabilty of potassium to displace the sodium from the plagioclase facilitating the growth of larger feldspar crystals.

A Consideration of plate configurations suggests a north dipping Variscan subduction zone. This type of collision would provide the granites on a palingenetic basis but the absence of many of the features associated with this type of oceanic-continental plate convergence, such as andesite-rhyolite-dacite volcanism casts doubt on this hypothesis.

The uncertainty as to the origin of the granites arises naturally from a lack of detailed knowledge about the deep geologic environment. An experimentally determined Strontium 87/86 ratio of 0.7067 for the St Austel granite (later modified to 0.7086 by a comutational method) only adds to the uncertainty, because this figure favours neither a crustal or mantle origin.