144 M. Godon’s mineralogical observations. 
composition, generally arising from the contact of water, which has 
less adherence to a smooth than to a rough surface. Hence some of 
the ancient porphyries, which were, so many centuries past, transport- 
ed from Asia to Greece, and from Greece to Italy, are still found 
among the ruins of this last country, sometimes in an unaltered 
state. 
Argilloid, 
29. The transitive point, which forms the limit between this rock 
and petrosilex, is yery uncertain; and these two rocks are often found 
uniting themselves in the same spot [Dorchester 0, Milton Ge Ke 
is highly probable they are formed of the same constituent parts, only 
with less cohesion in the argilloid. A property, which seems char- 
acteristic of this rock, is its tendency, when it exists in large body, 
to fall into thin plates; whence the vulgar name of slate commonly 
given to this mineral. 
Its common colour is grey, [Roxbury 0, Brighton oJ, greenish 
[Newton uJ, blackish [Weymouth mt, Goyernor’s island mJ; also 
brown, reddish brown [Malden x, Dorchester o]. Avvariety is found 
presenting thin white strata, in alternate order with brown red ones 
[Dorchester mn]. This last kind has the property of being used as 
whetstone, and may replace the Turkey-stone. The mineral, of both 
colours, has the same property. 
I distinguish only two species in this rock,—the common argilloid, 
the most common kind, found in Brighton, Newton, Roxbury ; and 
the novacular, that which may be used as whetstone. 
30, This» mineral often includes sulphurated iron ; some frag- 
ments are found presentin g veins of compact epidote, or compact car- 
bonated lime ; and others exhibit little fissures, in which a multitude 
