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SYMPOSIUM PHOTO INTERPRETATION, DELFT 1962 
ductivity values favour surface spalling, high values, more penetrative dis 
organisation. Planes of foliation or schistosity affect heat conduction and may 
render certain metamorphic rocks more resistant. 
The influence of rock texture is great. Differential stress is favoured by 
variety of mineral species, by unequal grain size and by a high proportion of 
large crystals. Equigranular monomineralic rocks tend to respond to thermal 
changes as a uniform whole and internal stress is minimised, as for example, in 
the case of metamorphic quartzite. In hyaline rocks thermal effects are largely 
restricted to surface flaking. 
Low porosity favours thermal disintegration. In plutonic rocks porosity is 
very low; it is significantly higher in certain metamorphic rocks, yet higher in 
many volcanic rocks. 
It is apparent that jointing affects the rate and course of weathering. The 
oretical studies of jointing propensity suggest that in plutonic rocks this in 
creases in the order diorite and gabbro, syenite, and granite. Intrusive granite 
is likely to be more affected by jointing than autochthonous granite. 
Surface colour has a marked affect on heat absorption but is largely over 
shadowed by conductivity properties. The colour of the fresh rock is generally 
modified by weathering. 
By evaluating major rock types with regard to the factors involved, tentative 
conclusions as to the order of response to thermal weathering may be reached. 
Plutonic igneous rocks are much affected; finer grained and more porous 
volcanic rocks are less responsive, and certain metamorphic rocks least sus 
ceptible. 
Among plutonic igneous rocks evidence consistently indicates granite as 
most affected. Lower in the scale is syenite, which in turn precedes diorite 
and gabbro. Virtually monomineralic dunite and pyroxenite are still more 
resistant. 
Rhyolitic rock is likely to be more responsive than andesite or basalt. 
Quartzite, monomineralic and equigranular when pure, is nevertheless more 
subject to thermal disintegration than crystalline limestone which, due partly 
to low thermal expansion properties, is little affected. 
Turning now to chemical weathering, in the arid regions referred to, 
evidence of chemical decay is clear. Lor example, many rocks are rotten and 
friable to considerable depths, felspars are kaolinised, olivine and nepheline 
decomposed, and decomposition products line invisible fracture planes to 
depths of three or more feet. Based on the established order of loss (and gain) 
of constituents during weathering, an order of chemical stability of minerals, 
and hence of rocks, can be developed. A point of importance is that the order 
of response to chemical weathering is at variance with that for thermal dis 
integration, and in the case of igneous rocks is approximately reversed. Acidic 
rocks are generally more resistant than basic rocks. Amorphous glassy rocks 
are readily attacked. High porosity and fine grain favour chemical decay but 
oppose thermal disintegration. 
In general, volcanic rocks are more subject to chemical attack than plutonic