270 Walk from Aberdeen 



the Plutonian rocks are granite, with felspar, or granitic por- 

 phyry. Besides these old rocks, there are also, although in 

 small quantity, members of the secondary class, viz. augite- 

 greenstone or dolerlte, and sandstone conglomerate. The Nep- 

 tunian primitive rocks exhibit numerous very interesting geo- 

 gnostical relations, all of which go to prove their chemical and 

 contemporaneous formation ; while the great bodies of Pluto- 

 nian granite, as those exposed in the celebrated granite quarries, 

 render it probable that the stratified Neptunian rocks owe much 

 of their contorted and broken aspect, and also, in some degree, 

 their position, to the action of this igneous rock. The most im- 

 portant of these primitive rocks, in an economical point of view, 

 is the granite, which is quarried ver3'^ extensively, and shipped 

 for many and distant parts of the island. The very coarse gra- 

 nular varieties of this rock form an indifferent building stone ; 

 whUe those varieties in which the granular concretions are of a 

 medium size, and well crystallized, and which resist a strong 

 pressure, aie the most valuable. The resistance which the dif- 

 ferent varieties around Aberdeen oppose to pressure is exceed- 

 ingly various ; the most esteemed kinds will bear, without being 

 crushed, on the square foot, a weight of 169,000 pounds ; Avhile 

 the best granite of Cornwall yields to a pressure of about 114,000 

 pounds. The only secondary Neptunian stratified rock we ob- 

 served at Aberdeen, is the conglomerate at the old bridge over 

 the Don. This conglomerate rests over the outgoings of the 

 inclined subjacent gneiss strata. It is composed of roundish 

 boulders, from the size of a musket-ball to that of a man''s head 

 and upwards, held together by a paste of a coarse and loosely 

 aggregated sandstone. The boulders are of granite, gneiss, 

 mica-slate, porphyry, hornblende-rock, quartz-rock inclining to 

 rock-crystal, with imbedded grains of felspar. The sandstone 

 forming the paste of the conglomerate consists of grains of 

 quartz, mica, and earthy felspar, which latter sometimes serves 

 as a basis for the other two ingredients. Thin layers of red- 

 coloured sandstone are visible in the conglomerate. The only 

 Plutonian secondarv rock was augite-greenstonc, or dolerite, 

 which occurs In veins or dikes cutting across the primitive 

 rocks. 



The liver Dee, alonjr whose banks wc strolled in our walk to 



