170 



HA RD WICKE'S SCIENCE- G OS SIP. 



tion of many exposures, and many scores of specimens 

 from various localities having failed to yield the 

 faintest signs of one. 



But though mica slate is the common rock of the 

 two counties, it is not persistent either in its texture 

 or in its mineralogical structure. It very frequently 

 passes into an argillaceous rock, having two distinct 

 forms, one a strict clay slate, in which the minerals 

 are quite unrecognisable, and the other a chlorite 

 slate. Both kinds are prevalent. In the latter the 

 mica has entirely disappeared, and has been replaced 

 by chlorite, giving to the mass a yellowish-green hue, 

 and a very greasy touch. A noticeable feature in 

 both kinds of rocks is the absence of quartz veins — 

 though this may be remarked of nearly all clayey 

 deposits. Most probably the rock was laid down 

 originally as a shale, which has since been indurated 

 by heat and pressure, a cause which at the same 

 time developed from the particles the chlorite which 

 occurs so abundantly in the mass. The general dip 

 of the strata is at an angle of not less than 20°, 

 though it very frequently approaches 30 . Every 

 precaution must be taken by the student that the 

 cleavage planes be not mistaken for the bedding 

 lines, an error easily fallen into where there are only 

 small local outcrops, and where but few bedding 

 lines are visible. Unfortunately no deep excavations 

 or very extensive cuttings have in any place been 

 made, the rock containing no mineral of economic 

 value, and being in itself of only minor importance 

 as a building-stone. For this purpose it is much too 

 liable to decomposition by weathering, whilst it is 

 generally quite useless as a roofing-slate on account 

 of its softness. When used for building purposes it 

 has, if exposed to the atmosphere, to be painted or 

 whitewashed, hence the peculiarly clean, not to say 

 picturesque, appearance of so many of the houses in 

 this part of Ireland. Of course this is the case with 

 many other dwellings in many other lands, and I 

 have frequently heard much praise and credit 

 bestowed upon the inhabitants on account of it. 

 But in reality, and with all due respect, encomiums 

 are no more due to them for these clean and neat 

 appearances than they are to the mole for making his 

 mound. Necessity demands a preservative for the 

 soft materials forming their houses. Necessity is the 

 mother of invention, and the invention in this instance 

 has been paint and whitewash. How frequently we 

 confound the results of necessity with the results of 

 virtue ! 



Mica slate is more commonly used than clay slate 

 or chlorite schist as a building material. It is much 

 harder and more durable. Berthier, who made a 

 special study of metamorphosed rocks, found the 

 first-named to contain ten per cent, more silica, and 

 two-and-a-half per cent, less water than the latter ; 

 an analysis which fully accounts for the difference in 

 the hardness of the respective materials. These 

 mica slates, clay slates, and chlorite slates are well 



exposed in the neighbourhood of Londonderry and 

 the south-west extremity of Lough Foyle, where 

 there are many very accessible exposures. 



Mica slate passing into gneiss occupies a much 

 smaller area, and may best be examined in Co. 

 Donegal to the west of Letterkenny. It probably 

 represents the shallow-water sandstones to which 

 allusion has previously been made. Here the gneiss 

 shows itself on the crests of the hills, giving to them 

 a more rugged aspect and a less fertile soil. This, 

 rock is composed of similar materials to those 

 forming the granite of the Derryveach Mountains — 

 quartz, ifelspar, and mica. It has a very massive 

 appearance and contains the first-named mineral in 

 grains the size of a small pea. These grains are also, 

 arranged in layers parallel to the bedding of the 

 rock. The matrix is quartz, though occasionally 

 very small crystals of felspar (orthoclase) are per- 

 ceptible in it. The mica becomes visible only on 

 the face of the layers. Quartz veins are numerous in- 

 this rock and appear to have been formed by the 

 infiltration of water containing silica in solution. 

 These cracks have been formed antecedent to the 

 metamorphic process, while the infiltration and 

 precipitation have quite probably been coincident 

 with it. Hand specimens do not exhibit the foliation 

 so commonly seen in gneiss, but the rock in the 

 mass shows it very clearly. 



At Buncrana on the east of Lough Swilly, and also- 

 extending from the west of the same Lough to 

 Kilcrennan, a most interesting series of metamorphic 

 limestones and quartzites occur. These are very 

 extensive, the latter being of a light red colour, and 

 the former varying from grey to green. The- 

 occasional bands of limestone and dolomite are very 

 similar to those found in the schistose beds of Scot- 

 land, and have been considered by many eminent 

 geologists to have been of contemporaneous origin. 

 North-east of Buncrana there is a trap dyke of 

 felspathic and augitic greenstone, which I am given 

 to understand finds much employment for those 

 individuals who are deprived of their liberty for 

 their country's good. 



Somewhere on or near to the eastern shore of 

 Lough Swilly there is a considerable bed of diailage 

 or gabbro rock — a crystallised aggregate of a variety 

 of augite with the triclinic felspar-plagioclase. The 

 first-named mineral appears in distinct dark-green 

 crystals, which are very lustrous. As I could only 

 find this rock in detached masses where the waters 

 of the Lough had evidently thrown them up, I am 

 unable to localise the bed. Perhaps some local 

 geologist may be able to do this. I, however, came 

 to the conclusion that it must be associated with 

 the same volcanic outburst which produced the 

 greenstone, and of which outburst it was probably 

 a part. 



Quartzite occurs at Mount Errigal (2460 feet), and 

 in many other places. It occupies a belt (occasionally 



