86 STRUCTURAL AND FIELD GEOLOGY 
The less acidic felspathic rocks, like the more acid types, are often 
fine-grained and compact—either sparingly porphyritic or without any 
conspicuous phenocrysts. The commonest varieties in this country are 
the andesites and porphyrites. These are more frequently dark-coloured 
than the highly acid rocks, but are on the whole lighter in colour than 
the basic rocks of which basalt may be taken as the type. Their 
hardness varies on fresh surfaces from 5:5 to 6; but as few of our 
andesites are without some alteration, their hardness is often less than 6, 
so that they can be scratched with a knife. The compact varieties are 
generally bluish or greenish, and tend to weather with a thinnish light- 
coloured crust, which is often more or less ferruginous (yellow or brown). 
But when they contain a considerable percentage of ferromagnesian 
minerals, they are darker coloured, and the crust is thicker and more 
markedly rusty in aspect. Varieties of this kind can hardly be dis- 
tinguished from similar fine-grained basalts, although there is almost 
always something in the general aspect of an andesite as seen in the 
mass, such as the character of its jointing and its mode of weathering, 
which, after some experience, the student will come to recognise. 
Compact phonolites are generally light-coloured—white, grey, bluish, 
or yellowish—and emit a bell-like clink when struck with the hammer. 
They are often readily split up into thin flags, and weather with a 
well-defined white crust. 
Fine-grained ¢rachyzes are commonly light or dark grey rocks, having 
the harsh or rough feel already referred to as more or less characteristic 
of some rhyolites—which they in this and other'respects closely resemble, 
They are differentiated from rhyolites chiefly by the absence of quartz. 
There is nothing in the mode of their weathering, however, to distinguish 
them from rhyolite. Neither trachytes nor phonolites are common rocks 
in Britain. Large crystals of sanidine are common in trachytes. 
Compact dzorvite varies in colour from greyish-green to dark green 
and black, and is known as aphanzfe—the green colour being due to its 
hornblende, and not necessarily to the presence of such decomposition- 
products as serpentine and chlorite. The fresh rock has a hardness of 
about 6. Such a rock closely resembles an amphibolite, from which, 
however, it may sometimes be distinguished by its weathering. If the 
rock be a diorite it will weather with a rusty crust, the inner portion of 
which (that, namely, which is nearest to the relatively unweathered rock) 
will exhibit effervescence with acid—thus revealing the presence of 
calcium carbonate—one of the products of the decomposition of the 
constituent felspar. 
Compact and fine-grained dasic rocks are widely distributed in this 
country. They are chiefly dasa//s, and in fresh exposures are very dark 
blue to black in colour. They have a hardness of 5 to 6, but when the 
rock is weathered the hardness may be much less. Very often a few 
isolated grains or an occasional granular aggregate of green or yellowish- 
green olivine may be seen, even in the most smoothly compact basalt. 
The jointing of the rock is generally more regular than that of the 
compact and fine-grained acid igneous rocks—basalt being frequently 

