CAPTAIN PORTLOCK'S REPORT ON THE GEOLOGY OF LONDONDERRY.
Tan volume owes its existence to an endeavour on the part ot some of the officers connected with the Ordnance Trigonometrical Survey of Ireland to engraft "statistical, antiquarian, and geogra- phical surveys" on the original plan. The idea was so far approved of as to be tolerated by the superior authorities; and directions were issued to the subordinates and assistants to collect materials for the after subjects, whilst occupied in the geographical survey. It was found in practice, however, that these secondary inquiries interfered with the essential object of a speedy production of the series of maps. After some modifications of the plan, the idea ap- pears to have been finally abandoned by the Board of Ordnance ; but Captain PORTLOCK received instructions to prepare for publi- cation the materials he had collected for the Geology of the county of Londonderry, and such parts of Tyrone and Fermanagh as were connected with Derry in a geological point of view. The very large and elaborate volume before us consists of this Report ; in the preparation of which, Captain PORTLOCK seems to have been less indebted to official encouragement, than to his own zeal, the authority of Colonel Coin, (the chief of the survey,) and the assistance of scientific friends.
As regards facts and information, little or nothing in this volume seems to be left for the geologist to desire. An Ordnance map, pre- pared to illustrate the geological strata, is attached to the work; besides wood-cuts interspersed throughout the text, nearly forty plates, with many specimens in each, exhibit the fossil remains ; and plans or smaller maps explain particular features—as the strata of a district, the workings of a mine. The Report itself is full almost to fatigue, unless to a geologist inquiring into some particular section ; and perhaps even to a geologist some parts might advantageously have been abbreviated. The in- troductory chapters on the different methods adopted by geologists in the signs and colours of mapping, with the plan chosen by the author—the review of the works of preceding writers on Irish geology — and the general observations on the characteristics and economical results of the physical features of a country— though by no means out of place, are scarcely essential at their present length, and might easily have been given in a more con- densed and attractive manner. The elaborate descriptions and tables of the geological " rocks " and detritus of the district sur- veyed, will be highly useful to any one interested in them theoreti- cally or practically ; but it is of a strictly scientific, perhaps of a technical character ; and so far as regards its mere utility, might have been presented with advantage in a more strictly technical form. In fact, the Report is of the nature of raw materials, highly desirable to be collected, perhaps desirable to be published, for it then takes its dance of utility, whereas when unpublished, useful it cannot be. Like many other official reports, however, it would have been more extensively useful had general views and selected facts—the essential points of the subject—been presented in a leading shape, and the subordinate details given in appendixes. As the Report stands, it is deficient in grasp.
The most popular section is that which relates to Economic Geology ; embracing facts on climate, cultivation, and so forth, ap- plied to practical purposes ; and the descriptive account of the collieries of Tyrone in the chapter on Carboniferous Strata. Much of this has a direct and obvious bearing ; though some of the sug- gestions for improving the agriculture and advancing the prosperity of Ireland are of rather a project-like character. From these more general passages we will take an extract or two.
IRISH CLIMATE AND ITS USES.
Of all subjects of conversation, the weather is perhaps the most engrossing; and in few countries is it so much a subject of complaint as in Ireland. Every one feels, acknowledges, and generally exaggerates the supposed evil; whilst the principal remedy of its practical ill consequences, drainage, is universally neglected. Too much water requires drainage as a corrective ; the excess of water being the evil, not the water itself. These remarks I make from the conviction which has often passed over my mind when visiting some of the wilder and more unpromising parts of the country, that the dampness of the climate, under the present rude system of tillage, is a blessing rather than a curse. It induces natural fertility, where without it there would be sterility; and appears to be the reason why soils of indifferent quality are so often covered with a fresh and wholesome verdure, and tracts of mountain or rocky ground, which would appear to repel cultivation, are made to produce crops and to support a considerable population. The wanderer along the mountain- side, or in such rocky districts as those of Orritor, Dungate, Craigballyharky, Cregganconroe, &c., will have frequent cause to admire the patience and forti- tude with which the humble peasant encounters the apparently impracticable task of clearing away stones, reclaiming bog, and introducing cultivation amidst the rocks: and though he may murmur at the frequent shower, to him so troublesome, he will readily recognize it as a powerful auxiliary to the poor man's labours. A dry climate would require a more perfect tillage to render the soil fitted to absorb moisture : a damp climate forces vege- tation in spite of indifferent culture. But if the climate in some degree favours the rude operations of the small farmer, it ought to stimulate him to exertion, by the conviction that were the ground drained and the culture improved, the chances of a good harvest would be vastly increased, even in mountain dis- tricts; and in the lower and more fertile tracts, where the excess of water has scarcely any counterbalancing advantage, a most serious evil would be re- moved.
LONDON AND DUBLIN WEATHER.
If the Dublin table be compared with that of London, several interesting results will be at once perceived. In Dublin, the average number of days of no rain is only 150, whilst in London it is 220: but at the same time the number of fair days is less in London ; so that the comparison would stand thus : No Rats Dublin 150 Loudon 220 Rainy. Heavy Rain. 94 24 82 21 8 12 3
The actual difference of the climate, as to the number of rainy days estimated on six years, is therefore 24 in favour of London ; the greater proportion of which falls into the class of partly wet and partly fair, the number of days of very heavy rain being nearly the same. The range of variation in the number of days of no rain was nearly equal in both Countries; but, combining no rain and fair, there was less variation in the climate of Dublin than in that of London.
No Rain. No Raiu and Fair. Dublin. London. Dublin. London. 1837 177 242 216 247 1841 120 188 197 205 — — — — 57 54 19 42 The number of days of heavy rain varies from 18 to 32 in Dublin, from 16 to 30 in London ; but it is remarkable, that the years do not in this respect cor-
respond; 1841 being the year of least heavy rain in Dublin, and 1839 that of the most ; 1837 the year of least, 1841 of most, in London ; and this difference, consequent on the different local position of the places, is also observable in the actual quantities of rain.
Fair. No Rain and Fair. 56 206 10 230
70 in favour of London. 46 ill favour of Dublin. 24 in favour of London. And in like manner—
Light Showers. Dublin 41 London 33
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