52 
THE GEOLOGIST. 
building tlie new Houses of Parliament_^ the following inference 
may be drawn^ that in all stones of the same class, the heaveisf 
Mud, or that which has the greatest specific gravity, is the most 
durahle and hest suited for architectural woi^ks. This is given as 
a general rule^ altbougb liable to some exceptions^ but it appears, 
to bold good witb all varieties of Portland stone. 
The beds of Portland stone are very different in quality, the 
most perfect being that which is situated about midway in the 
series. The stratification varies in thickness from north to south ; 
this is very conspicuous in the Way-croft quarries, where the top 
bed is double the thickness of the corresponding bed in the Grove 
quarries; the former is perhaps the best stone in the island, 
whereas the latter is so corse and inferior that it is rarely or 
never used. 
With respect to the sandstones generally it is essential to re- 
mark that as laminated strata, blocks cut from them must par- 
talte of their nature, and consequently, if j^laced in an exposed 
situation with the planes af lamination parallel to the outward 
surface of the building, it will exfoliate or decompose in flakes, 
according to the thickness of the laminae, whereas if the same 
stone were placed so that the plane of lamination be horizontal, 
that is most commonly upon its natural bed, the amount of de- 
composition will be comparatively immaterial. 
Some of the sandstones are iregularly laminated, presenting 
an appearance across the strata of a number of short black lines, 
arising from small quantities of carbonaceous matter being 
interpersed in beds, often parallel to each other, as in Craigleath 
bed rock, but occasionally as if formed by a succession of minute 
undulations, similar to rippled markings of agitated shallow 
water upon sand. The stone from Garscube or President quaiTy, 
near Glasgow, is of this description. 
Many sandstones are composed of small pebbles varying con- 
siderably in appearance, and often differing essentially in their 
elementary character. These arc coarsely Gonglomorated to- 
