HARD WICKE'S SCIENCE- GOSSIP. 



that strew the slopes and beds of the valleys of the 

 islands, are derived. 



The action of the sirocco and the rain upon the 

 sand-bed that serves as the foundations of the forma- 

 tion, by gradually wearing it away, thus deprives the 

 upper bed of its support, and causes the cliffs to 

 break away in cyclopean masses, and to strew the 

 slopes of the hills and valleys with their debris ; while 

 other masses are detached and are tilted so perilously 

 out of the perpendicular that they appear — 



" As if an infant's touch could urge 

 Its headlong passage down the verge." 



Such are a few of the effects 'that this powerful 

 eroding agent is, in part, accountable for ; but it has 

 been assisted in its work by other and equally 

 powerful auxiliaries, without whose co-operation its 

 efforts could not have been so effective. The main 

 features of the country, the hills, valleys, and gorges 

 have had their direction and extent largely influenced 



and on every rock, boulder, or other rock-surface. 

 The irregular blocks of which the walls that serve 

 as boundary -partitions between the fields, and the 

 tooled stones of which the edifices in the towns and 

 casals are built afford equally striking evidences of its 

 powers of erosion ; and by their means both the rate 

 and the amount of the denudation may be estimated. 

 It is a noteworthy feature in the exteriors of Maltese 

 walls and houses that the side that is exposed to the 

 sirocco always presents a very eroded, time-worn and 

 dilapidated appearance, whereas the other sides, in 

 comparison, are fresh and unworn. 



It is no uncommon occurrence to find the softer 

 stones in the sides of the houses that have a south-east 

 aspect, almost completely worn through, and sur- 

 rounded by other blocks, the harder portions of which 

 such as the fossil contents, echinoides, pectens, etc., 

 stand out in bold relief from their worn and wasted 

 matrices. In the old fortifications that were erected 

 by the Knights of St. John, such phenomena as these 



'^y^^^^^^^ :: - 



Fig. 3. — Effects of Sirocco abrasion on ruins. 



by the lay of the strata ; while the minor ones, such 

 as the honey-combed and fretted appearances presented 

 by the cliff-faces and rock-surfaces, have been in- 

 fluenced by the lithological characters of the rock. 

 These are some of the assistants that have co-operated, 

 add to which the heat and drought of summer, and 

 the wet and cold of winter. 



But effective as they are as helpers in the work of 

 waste, no single one of them can be pointed to as 

 being more potent, more active, more irresistible than 

 the sirocco. 



Both in Malta and in Gozo the principal valleys 

 lay in a north-west and a south-east direction ; that is 

 to say, they lie in a line with the direction of this 

 wind. 



Marsa Sirocco, an extensive bay on the east coast 

 of Malta, so called because this wind blows directly 

 into it, owes its origin and extent to its agency. It is 

 the largest bay in the islands, and has four valleys 

 abutting on its coast-line, each of which lies in the 

 same direction. But it is not only in the general 

 moulding of the country that the sirocco is concerned. 

 Its effects may be traced in every crag and cavern, 



are of frequent occurrence, and are very typical of 

 sirocco denudation. 



From a series of calculations that I have made of 

 the rate of the erosion of the Globigerina limestone 

 blocks in a number of buildings and fortifications of 

 known ages, I estimate that the rate of sirocco 

 denudation averages $ z of an inch per square foot per 

 year ; that is about 16 cubic yards per acre per year ; 

 or about 22 tons of material are annually wasted from 

 every acre of surface. 



In calculating this, numerous examples were taken, 

 some being in proximity to" the coast, while others 

 were obtained from the centres of both islands. By 

 so doing I believe I have obtained a fair average 

 rate, for there can be no doubt, but that the rate of 

 erosion is more rapid near the coast than it is inland. 

 The moisture-ladened winds that sweep over the 

 islands impregnate all that they come in contact 

 with ; and the Globigerina rock being very porous, 

 is therefore highly susceptible to its influence. 



The duration of time during which the sirocco 

 lasts is seldom long enough to enable it to do more 

 than affect the surface, and then the period of 



