Decembek 1, 1900.] 



KNOWLEDGE 



273 



cut along vertical cliffs of what seems the most 

 tiTacherous material, xiutil it can find a firmer hold ou 

 the Jurassic or Cretaceous rock beyond. 



Some of the old pebbly alluvium, however, has become 

 consolidated as a firm couglomcrate, browu and massive, 

 breaking now into huge rectangular blocks. It is diffi- 

 cult to believe that this rock is of about the same age 

 as our glaciaJ gi'avcls in the British Isles. It forms 

 solid platlorms in the valley, on one of which the town 

 of Embrun stands ; ou another, still more imposing, 

 the fortress of Mont Dauphin has been piled. This 

 conglomerate has its analogue in the beds that once 

 choked the Alpine streams, as tliey emerged on Bavaria^I 

 or on the plain of Italy. 



The decay of Dauphine under the tremendous battery 

 of its storms is aided by the nature of the Mesozoic 

 rocks themselves. All down the valley of the Durance, 

 a large part of these consist of black shales, nearly 

 as hard as slate, which yet crumble up into mere flaky 

 powder when exposed. In these beds, the occasional 

 thin bands of yellowish limestone alone mark the strati- 

 fication. Being much contorted, they run like conven- 

 tional streaks of lightning across the uniform blackness 

 of the shales, surprising us again and again by their 

 evidence of the original structm'e of the mass. In 

 between them, and often involving them in decay, the 

 shale weathers down like a mere rubbish-heap exjiosed 

 to the wash of I'ain. Characteristic little cirques are 

 worked out, by the union of the rivulets that occur 

 during storms and rapidly die away again. Each group 

 of these rivulets terminates in a common channel below, 

 and the rock-mass between two adjacent channels in time 

 comes to stand out like a steep conical earth-pillar or bluff. 

 Whole hillsides are cut up into these pillars or earth- 

 pyramids, as if the material was some superficial 

 moraine, instead of the rock that build.i the moun- 

 tains. Probably, the alternations of storm and intense 

 sunlight — for we are here on the latitude of Florence — 

 have much to do with the rapidity and imiformity 

 of disintegration. Compact shales, that in oiu- insular 

 climat« might weather out almost like the slates of 

 Moelwyn, are here doomed to destruction before oiu' 

 eyes. The pyramidal or steepy conical forms due to 

 denudation become fascinating, and at the same time 

 monotonous. In the wild ravine from Savines down to 

 Espinasses, we lose sight of them with pleasure ; lime- 

 stone here predominates in the Jurassic series, and 

 sheer rock-walls and terrace-structures aix the result. 

 But, at the further end, the fantastic forms return t-o 

 us like an evil dream ; at times the outer sides of the 

 bluffs become rounded, resembling badly made columns, 

 or the swollen trunks of trees. A vision of elephan- 

 tiasis presents itself ; whole mountains appear to be 

 abnormally diseased. The rotten condition of the surface 

 is seen when one of the larger side-streams cuts its way 

 down to the Durance ; the ravine made by it in the 

 black strata repeats the characters of those carved in 

 loose alluvium. 



This valley of the Durance provides, indeed, an 

 amazing picture of destruction. We begin to ask our- 

 selves, what is solid, what is proof against the thunder- 

 storm that crashes through the mountains, or against 

 the dry heat, the glare of southern sunlight, that strikes 

 up towards us from Provence? The enormous cliffs of 

 massive limestone that at last rise around us, pale and 

 ghostly in the blackness of the storm, may give us some 

 assurance; they bring south, as a bulwark to Dauphine, 

 the finest features of the Juras. But our impressions 



^ See KKOwiEDaB, Tol. XXIII., p. 123. 



of the Durance, down to the dcltarformations among 

 the almond-orchards of Manosque, arc those of a stream 

 pillaging a country. The dust that forms on the 

 disintegrating surfaces in days of burning sunshine 

 spreads itself to the leaves of the vines and to the grass 

 of shadele-ss fields. Even the tiled roofs of the villages 

 have caught the prevailing dust-colour, a uniform tone 

 of yellow earth. The bastidc of Montfort is yellow on 

 its yellow hill ; the hamlets along the mountain-spurs 

 conceal themselves successfully, by mimicry of the stonc- 

 hcaps from which they rose. These contrasts between 

 the crumbling foothills and the old Alpine core which 

 has become pre-eminent by their decay are nowhere more 

 brilliantly revealed than here on the outskirts of Pro- 

 vence. For always behind us we may catch some glimpse 

 of the granite heart of Dauphine; beyond the long 

 valley, and the ruin of its walls, lies the glory of the 

 high alp and the snows. 



♦ 



THE MILKY WAY IN CYGNUS. 



By Mrs. W.\lter Maunder. 



The accompanying photograph was taken in the second 

 week of August, 1S99, during the progress of the Perseid 

 shower. The photographic object glass used was a Dall- 

 meyer stigmatic lens of 1^ inches aperture and 9 inches 

 focal length. The size of the plate is that standardized 

 by the International Astrographic Chart, namely, 16 

 centimetres square, so that the area of the sky covered 

 is about 1400°. The camera was mounted on an 

 ecjuatorial stand, constructed by the late Mr. Sydney 

 Waters, f.r.a.s., for use in eclipse observation, and 

 bequeathed by him to the Royal Astronomical Society, 

 by whom it was lent to my husband. It was housed in 

 a small wooden observatory, made with a roof that could 

 slide quits away and leave as much sky room available 

 as the other conditions of the situation allowed. This 

 was not very great, as the garden in which the obser- 

 vatory was placed was small, and nuuh surrounded by 

 houses, which not only limited the horizon, but abounded 

 with windows that were unpleasantly illuminated at 

 night. 



The photograph was exposed for G| hours, but the 

 exposure was distributed over several nights, and this 

 for various reasons. In the first place, the equatorial 

 was of the " German " form, i.e., there was a counter- 

 poise to balance the camera at the other end of the 

 declination axis. Consequently either the camera or 

 the countei-poise would foul the stand shortly after 

 passing the meridian, and the instrument would havo 

 to be '■ reversed " in order to continue to follow the stars. 

 This would also mean reversing the relative jjositions of 

 the stars in the telescope, which in visual work would not 

 matter, since the eye does not retain on the retina the 

 first positions of the stars, but which in the camera 

 would introduce new and unknown configurations of stars 

 on the plate. And the exposure must not take pla,ce 

 whilst the guiding star is very far distant from its 

 meridian, since its altitude is then rapidly changing, 

 and the area covered by the plate b;ing very considerable 

 the effect of refraction would vary largely over the plate, 

 and be, moreover, variable in its change. Thirdly, and 

 this was by no means the least important factor, the 

 strain ou the observer was very groat. The guiding 

 telescope was a Cooke refractor of 2A inches aperture, 

 small but of excellent quality; the diffraction rings 

 round the brighter stars were many and almost perfect. 

 There were absolutely no luxuries in the shape of 

 illumination of field or wires, not to speak of electric 

 control to the driving clock; the observer had to bo 



